Backlight and display system using same

ABSTRACT

A backlight that includes a front reflector and a back reflector that form a hollow light recycling cavity including an output surface is disclosed. The backlight further includes one or more light sources disposed to emit light into the light recycling cavity. The front reflector includes an on-axis average reflectivity of at least 90% for visible light polarized in a first plane, and an on-axis average reflectivity of at least 25% but less than 90% for visible light polarized in a second plane perpendicular to the first plane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. §371 ofPCT/US2008/064133, filed on May 19, 2008, which claims priority to U.S.Provisional Application No. 60/939,079, filed on May 20, 2007, thedisclosure of which is incorporated by reference in its/their entiretyherein.

RELATED APPLICATIONS

The following co-owned and copending PCT Patent Applications areincorporated herein by reference: PCT Patent Publication No.WO2008/144636; PCT Patent Publication No. WO2008/144644; PCT PatentPublication No. WO2008/147753; and PCT Patent Publication No. WO2008/144650.

FIELD

The present disclosure relates to extended area light sources suitablefor illuminating a display or other graphic from behind, commonlyreferred to as backlights. The disclosure is particularly suited, butnot necessarily limited, to backlights that emit visible light ofsubstantially only one polarization state.

BACKGROUND

Historically, simple backlight devices included only three maincomponents: light sources or lamps, a back reflector, and a frontdiffuser. Such systems are still in use for general purpose advertisingsigns and for indoor lighting applications.

Over recent years, refinements have been made to this basic backlightdesign by adding other components to increase brightness or reduce powerconsumption, increase uniformity, and/or reduce thickness. Therefinements have been fueled by demands in the high-growth consumerelectronics industry for products that incorporate liquid crystaldisplays (LCDs), such as computer monitors, television monitors, mobilephones, digital cameras, pocket-sized MP3 music players, personaldigital assistants (PDAs), and other hand-held devices. Some of theserefinements, such as the use of solid light guides to allow the designof very thin backlights, and the use of light management films such aslinear prismatic films and reflective polarizing films to increaseon-axis brightness, are mentioned herein in connection with furtherbackground information on LCD devices.

Although some of the above-listed products can use ordinary ambientlight to view the display, most include a backlight to make the displayvisible. In the case of LCD devices, this is because an LCD panel is notself-illuminating, and thus is usually viewed using an illuminationassembly or backlight. The backlight is situated on the opposite side ofthe LCD panel from the viewer, such that light generated by thebacklight passes through the LCD to reach the viewer. The backlightincorporates one or more light sources, such as cold cathode fluorescentlamps (CCFLs) or light emitting diodes (LEDs), and distributes lightfrom the sources over an output area that matches the viewable area ofthe LCD panel. Light emitted by the backlight desirably has sufficientbrightness and sufficient spatial uniformity over the output area of thebacklight to provide the user with a satisfactory viewing experience ofthe image produced by the LCD panel.

LCD panels, because of their method of operation, utilize only onepolarization state of light, and hence for LCD applications it isimportant to know the backlight's brightness and uniformity for light ofthe correct or useable polarization state, rather than simply thebrightness and uniformity of light that may be unpolarized. In thatregard, with all other factors being equal, a backlight that emits lightpredominantly or exclusively in the useable polarization state is moreefficient in an LCD application than a backlight that emits unpolarizedlight. Nevertheless, backlights that emit light that is not exclusivelyin the useable polarization state, even to the extent of emittingrandomly polarized light, are still fully useable in LCD applications,since the non-useable polarization state can be easily eliminated by anabsorbing polarizer provided at the back of the LCD panel.

LCD devices generally fall within one of three categories, andbacklights are used in two of these categories. In a first category,known as “transmission-type,” the LCD panel can be viewed only with theaid of an illuminated backlight. That is, the LCD panel is configured tobe viewed only “in transmission,” with light from the backlight beingtransmitted through the LCD on its way to the viewer. In a secondcategory, known as “reflective-type,” the backlight is eliminated andreplaced with a reflective material, and the LCD panel is configured tobe viewed only with light sources situated on the viewer-side of theLCD. Light from an external source (e.g., ambient room light) passesfrom the front to the back of the LCD panel, reflects off of thereflective material, and passes again through the LCD on its way to theviewer. In a third category, known as “transflective-type,” both abacklight and a partially reflective material are placed behind the LCDpanel, which is configured to be viewed either in transmission if thebacklight is turned on, or in reflection if the backlight is turned offand sufficient ambient light is present.

Backlights described in the detailed description below can generally beused both in transmission-type LCD displays and in transflective-typeLCD displays.

Besides the three categories of LCD displays discussed above, backlightscan also fall into one of two categories depending on where the internallight sources are positioned relative to the output area of thebacklight, where the backlight “output area” corresponds to the viewablearea or region of the display device. The “output area” of a backlightis sometimes referred to herein as an “output region” or “outputsurface” to distinguish between the region or surface itself and thearea (the numerical quantity having units of square meters, squaremillimeters, square inches, or the like) of that region or surface.

In “edge-lit” backlights, one or more light sources are disposed—from aplan-view perspective—along an outer border or periphery of thebacklight construction, generally outside the area or zone correspondingto the output area. Often, the light source(s) are shielded from view bya frame or bezel that borders the output area of the backlight. Thelight source(s) typically emit light into a component referred to as a“light guide,” particularly in cases where a very thin profile backlightis desired, as in laptop computer displays. The light guide is a clear,solid, and relatively thin plate whose length and width dimensions areon the order of the backlight output area. The light guide uses totalinternal reflection (TIR) to transport or guide light from theedge-mounted lamps across the entire length or width of the light guideto the opposite edge of the backlight, and a non-uniform pattern oflocalized extraction structures is provided on a surface of the lightguide to redirect some of this guided light out of the light guidetoward the output area of the backlight. (Other methods of gradualextraction include using a tapered solid guide, where the sloping topsurface causes a gradual extraction of light as the TIR angle is, onaverage, now reached by greater numbers of light rays as the lightpropagates away from the light source.) Such backlights typically alsoinclude light management films, such as a reflective material disposedbehind or below the light guide, and a reflective polarizing film andprismatic Brightness Enhancement Films (BEF) film(s) disposed in frontof or above the light guide, to increase on-axis brightness.

In the view of Applicants, drawbacks or limitations of existing edge-litbacklights include the following: the relatively large mass or weightassociated with the light guide, particularly for larger backlightsizes; the need to use components that are non-interchangeable from onebacklight to another, since light guides must be injection molded orotherwise fabricated for a specific backlight size and for a specificsource configuration; the need to use components that requiresubstantial spatial non-uniformities from one position in the backlightto another, as with existing extraction structure patterns; and, asbacklight sizes increase, increased difficulty in providing adequateillumination due to limited space or “real estate” along the edge of thedisplay, since the ratio of the circumference to the area of a rectangledecreases linearly (1/L) with the characteristic in-plane dimension L(e.g., length, or width, or diagonal measure of the output region of thebacklight, for a given aspect ratio rectangle).

In “direct-lit” backlights, one or more light sources are disposed—froma plan-view perspective—substantially within the area or zonecorresponding to the output area, normally in a regular array or patternwithin the zone. Alternatively, one can say that the light source(s) ina direct-lit backlight are disposed directly behind the output area ofthe backlight. Because the light sources are potentially directlyviewable through the output area, a strongly diffusing plate istypically mounted above the light sources to spread light over theoutput area to veil the light sources from direct view. Again, lightmanagement films, such as a reflective polarizer film, and prismatic BEFfilm(s), can also be placed atop the diffuser plate for improved on-axisbrightness and efficiency. Large area LCD applications tend to usedirect-lit backlights because they are not constrained by the 1/Llimitation of edge-lit backlights and because of the weight associatedwith solid light guides.

In the view of Applicants, drawbacks or limitations of existingdirect-lit backlights include the following: inefficiencies associatedwith the strongly diffusing plate; in the case of LED sources, the needfor large numbers of such sources for adequate uniformity andbrightness, with associated high component cost and heat generation; andlimitations on achievable thinness of the backlight beyond which lightsources produce non-uniform and undesirable “punchthrough,” where abright spot appears in the output area above each source.

In some cases, a direct-lit backlight may also include one or some lightsources at the periphery of the backlight, or an edge-lit backlight mayinclude one or some light sources directly behind the output area. Insuch cases, the backlight is considered “direct-lit” if most of thelight originates from directly behind the output area of the backlight,and “edge-lit” if most of the light originates from the periphery of theoutput area of the backlight.

SUMMARY

In one aspect, the present disclosure provides a backlight that includesa front reflector and a back reflector that form a hollow lightrecycling cavity including an output surface. The backlight alsoincludes one or more light sources disposed to emit light into the lightrecycling cavity. The front reflector includes an on-axis averagereflectivity of at least 90% for visible light polarized in a firstplane, and an on-axis average reflectivity of at least 25% but less than90% for visible light polarized in a second plane perpendicular to thefirst plane.

In another aspect, the present disclosure provides an asymmetricreflective film including alternating polymer layers whose arrangementand refractive indices are tailored to provide an average on-axisreflectivity of at least 90% for visible light polarized in a firstplane, and an average on-axis reflectivity of at least 25% but less than90% for visible light polarized in a second plane perpendicular to thefirst plane.

In another aspect, the present disclosure provides a display system thatincludes a display panel, and a backlight disposed to provide light tothe display panel. The backlight includes a front reflector and a backreflector that form a hollow light recycling cavity including an outputsurface. The backlight further includes one or more light sourcesdisposed to emit light into the light recycling cavity. The frontreflector includes an on-axis average reflectivity of at least 90% forvisible light polarized in a first plane, and an on-axis averagereflectivity of at least 25% but less than 90% for visible lightpolarized in a second plane perpendicular to the first plane.

In another aspect, the present disclosure provides a backlight thatincludes a front reflector and a back reflector that form a hollow lightrecycling cavity including an output surface. The front reflector isconfigured to reflect substantially all light within the cavity that hasa first angular distribution and to partially reflect and partiallytransmit light within the cavity having a second angular distributionthat is different than the first angular distribution. The backlightfurther includes one or more light sources disposed to emit light intothe cavity, and a conversion structure positioned within the cavity toconvert at least a portion of light in the cavity having the firstangular distribution into light having the second angular distributionand at least a portion of light in the cavity having the second angulardistribution into light having the first angular distribution.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification, reference is made to the appendeddrawings, where like reference numerals designate like elements, andwherein:

FIG. 1 is a schematic cross-section view of one embodiment of adirect-lit display system.

FIG. 2 is a schematic cross-section view of one embodiment of anedge-lit backlight.

FIG. 3 is a schematic perspective view of one embodiment of a multilayeroptical film.

FIG. 4 is a schematic perspective view of a reflective polarizing film.

FIG. 5 is a graph of reflectivity versus incidence angle in air for oneembodiment of an asymmetric reflective film.

FIG. 6 is a schematic cross-section view of one embodiment of a portionof a hollow light recycling cavity.

FIG. 7A is a graph of reflectivity versus incidence angle in air foranother embodiment of an asymmetric reflective film.

FIG. 7B is a graph of reflectivity versus incidence angle in air foranother embodiment of an asymmetric reflective film.

FIGS. 8A-C are schematic cross-section views of various embodiments offront reflectors.

FIG. 9A is a schematic view of a method useful for the coextrusion ofasymmetric reflective films.

FIG. 9B is a schematic perspective view of one embodiment of a feedblockthat can be used in the method illustrated in FIG. 9A.

FIG. 10 is a graph of transmissivity versus wavelength of an asymmetricreflective film formed using the method illustrated in FIG. 9A.

FIG. 11 is a schematic cross-section view of a portion of one embodimentof a backlight that includes a diffusely reflective front reflector anda diffusely reflective back reflector.

FIG. 12 is a schematic cross-section view of a portion of one embodimentof a backlight that includes a specularly reflective front reflector anda semi-specular back reflector.

FIG. 13 is a graph of the fractional output of a cavity versus 1 minusthe Cavity Loss value for front reflectors with various on-axis averagereflectivities for light polarized parallel to the pass axis of thefront reflector.

FIG. 14 is a graph of the on-axis polarized output gain versus thereflectivity of a front reflector for various values of % backlightloss.

FIG. 15 is a schematic cross-section view of one embodiment of adirect-lit backlight.

FIG. 16 is a schematic cross-section view of another embodiment of adirect-lit backlight.

FIG. 17 is a schematic plan view of one embodiment of a zoned backlight.

FIG. 18 is a schematic view of the approximate dependence of the totalreflectivity upon the direction of incidence for one or more embodimentsof front reflectors.

FIG. 19 is a graph of luminance versus position for several embodimentsof front reflectors in an edge-lit backlight.

FIGS. 20-27 are graphs of luminance versus position for severalembodiments of front reflectors in an edge-lit backlight.

FIG. 28 is a schematic cross-section view of one embodiment of aprismatic brightness enhancing film.

FIGS. 29-35 are graphs of luminance versus viewing angle for severalembodiments of front reflectors as measured using a gain cube.

DETAILED DESCRIPTION

In general, the present disclosure describes backlights that providebrightness and spatial uniformity that are adequate for the intendedapplication. Such backlights can be used for any suitable lightingapplication, e.g., displays, signs, general lighting, etc. In someembodiments, the described backlights include a hollow light guideformed by a front reflector and a back reflector. The front reflectorcan be partially transmissive, thereby allowing emission of light havinga desired optical characteristic or combination of opticalcharacteristics. In some embodiments, the desired optical characteristiccan include a selected polarization state; in other embodiments, thedesired optical characteristic can include emitted light having selectedviewing angles.

In exemplary embodiments, the disclosed backlights balance the followingcharacteristics: 1) the amount of recycling of a desired polarizationstate; 2) the degree of scattering of light within the cavity; and 3)the angular and spatial distribution of light directed into the cavity.This balancing/tailoring can provide substantial filling of the cavitywith light (both spatially and angularly) using recycling and optionallya controlled amount of diffusion. The amount of recycling is sufficientto achieve the desired backlight uniformity with minimum degradation ofbacklight efficiency and brightness. This balancing can also providebacklights whose brightness and uniformity are compatible with highperformance display applications, but where the backlights also havepreviously unachievable physical proportions (e.g., low profile design)or optical properties (e.g., large output area for given light sourceemission area).

In some embodiments, this balancing is attained by using frontreflectors that have an intermediate on-axis average reflectivity in thepass state. In exemplary embodiments, the front reflector has an on-axisaverage reflectivity of at least 90% for visible light polarized in afirst plane, and an on-axis average reflectivity of at least 25% butless than 90% for visible light polarized in a second planeperpendicular to the first plane.

In conventional backlights, the bulb-to-diffuser spacing, thebulb-to-bulb spacing, and the diffuser transmission are significantfactors to be considered in designing the backlight for a given value ofbrightness and illumination uniformity. Generally, a strong diffuser,i.e., a diffuser that diffuses a greater fraction of the incident light,improves the uniformity but results in reduced brightness because thehigh diffusing level is accompanied by strong back diffusion, i.e.,reflection. Such strong diffusers can also increase the overallthickness profile of the backlight.

According to some embodiments of the present disclosure, the partiallytransmissive front reflector may provide greater illuminance uniformityand/or color mixing without the need for a strong diffuser, therebydecreasing the thickness profile of the backlight.

In embodiments where the backlight includes light sources capable ofproducing light having different peak wavelengths or colors (e.g., anarray of red, green, and blue LEDs), the high-recycling cavity isoperable to distribute the light such that the light directed out of thedevice is more uniform in color and intensity. For example, when whiteillumination light is desired, the disclosed cavities can mix light fromindividually colored light sources such that the appearance at the LCpanel is of a more uniform white light. Such recycling cavities can besignificantly thinner than standard backlights used, e.g., in LCdisplays.

The backlights of the present disclosure can be utilized as backlightsfor display systems, e.g., LC displays; however, the backlights asdescribed herein are not restricted to use for illuminating a liquidcrystal display panel. The disclosed backlights may also be usedwherever discrete light sources are utilized to generate light, and itis desirable to have uniform illumination out of a panel that includesone or more of the discrete light sources. Thus, the describedbacklights may be useful in solid state space lighting applications,signs, illuminated panels, and the like.

In general, it would be beneficial for next generation backlights tocombine some or all of the following characteristics while providing abrightness and spatial uniformity that is acceptable for the intendedapplication: thin profile; design simplicity, such as a minimal numberof film components and a minimal number of sources, and convenientsource layout; low weight; no use of or need for film components havingsubstantial spatial non-uniformities from one position in the backlightto another; compatibility with LED sources; insensitivity to problemsassociated with color variability among LED sources that are allnominally the same color; to the extent possible, insensitivity to theburnout or other failure of a subset of LED sources; and the eliminationor reduction of at least some of the limitations and drawbacks mentionedin the Background section above.

Whether these characteristics can be successfully incorporated into abacklight depends in part on the type of light source used forilluminating the backlight. CCFLs, for example, provide white lightemission over their long narrow emissive areas, and those emissive areascan also operate to scatter some light impinging on the CCFL, such aswould occur in a recycling cavity. The typical emission from a CCFL,however, has an angular distribution that is substantially Lambertian,which may be inefficient or otherwise undesirable in a given backlightdesign. Also, the emissive surface of a CCFL, although somewhatdiffusely reflective, also typically has an absorptive loss thatApplicants have found to be significant if a highly recycling cavity isdesired.

An LED die emits light in a near-Lambertian manner, but because of itsmuch smaller size relative to CCFLs, the LED light distribution can bereadily modified, e.g., with an integral encapsulant lens, reflector, orextractor to make the resulting packaged LED a forward-emitter, aside-emitter, or other non-Lambertian profile. Such profiles can provideimportant advantages for the disclosed backlights. However, the smallersize and higher intensity of LED sources relative to CCFLs can also makeit more difficult to produce a spatially uniform backlight output usingLEDs. This is particularly true in cases where individually coloredLEDs, such as arrangements of red/green/blue (RGB) LEDs, are used toproduce white light, since failure to provide adequate lateral transportor mixing of such light can result in undesirable colored bands orareas. White light emitting LEDs, in which a phosphor is excited by ablue or UV-emitting LED die to produce intense white light from a smallarea or volume on the order of an LED die, can be used to reduce suchcolor non-uniformity. But white LEDs currently are unable to provide LCDcolor gamuts as wide as those achievable with individual colored LEDarrangements and thus may not be desirable for all end-use applications.

Applicants have discovered combinations of backlight design featuresthat are compatible with LED source illumination, and that can producebacklight designs that outperform backlights found in state-of-the-artcommercially available LCD devices in at least some respects. Thesebacklight design features include some or all of the following:

-   -   A. a recycling optical cavity in which a large proportion of the        light undergoes multiple reflections between substantially        coextensive front and back reflectors before emerging from the        front reflector, which is partially transmissive and partially        reflective;    -   B. overall losses for light propagating in the recycling cavity        are kept extraordinarily low, for example, both by providing a        substantially enclosed cavity of low absorptive loss, including        low loss front and back reflectors as well as side reflectors,        and by keeping losses associated with the light sources very        low, for example, by ensuring the cumulative emitting area of        all the light sources is a small fraction of the backlight        output area;    -   C. a recycling optical cavity that is hollow, i.e., the lateral        transport of light within the cavity occurs predominantly in        air, vacuum, or the like rather than in an optically dense        medium such as acrylic or glass;    -   D. in the case of a backlight designed to emit only light in a        particular (useable) polarization state, the front reflector has        a high enough reflectivity for such useable light to support        lateral transport or spreading, and for light ray angle        randomization to achieve acceptable spatial uniformity of the        backlight output, but a high enough transmission into the        appropriate application-usable angles to ensure application        brightness of the backlight is acceptably high;    -   E. the recycling optical cavity contains a component or        components that provide the cavity with a balance of specular        and diffuse characteristics, the component having sufficient        specularity to support significant lateral light transport or        mixing within the cavity, but also having sufficient diffusivity        to substantially homogenize the angular distribution of steady        state light within the cavity, even when injecting light into        the cavity only over a narrow range of propagation angles.        Additionally, recycling within the cavity can result in a degree        of randomization of reflected light polarization relative to the        incident light polarization state. This allows for a mechanism        by which unusable polarization light can be converted by        recycling into usable polarization light;    -   F. the front reflector of the recycling cavity has a        reflectivity that generally increases with angle of incidence,        and a transmission that generally decreases with angle of        incidence, where the reflectivity and transmission are for        unpolarized visible light and for any plane of incidence, and/or        for light of a useable polarization state incident in a plane        for which oblique light of the useable polarization state is        p-polarized. Additionally, the front reflector has a high value        of hemispheric reflectivity, and simultaneously, a sufficiently        high value of transmission of application usable light;    -   G. light injection optics that partially collimates or confines        light initially injected into the recycling cavity to        propagation directions close to a transverse plane (the        transverse plane being parallel to the output area of the        backlight), e.g., an injection beam having a full angle-width        (about the transverse plane) at half maximum power (FWHM) in a        range from 0 to 90 degrees, or 0 to 60 degrees, or 0 to 30        degrees. In some instances it may be desirable for the maximum        power of the injection light to have a downward projection,        below the transverse plane, at an angle with the transverse        plane of no greater than 40 degrees, and in other instances, to        have the maximum power of the injected light to have an upwards        projection, above the transverse plane towards the front        reflector, at an angle with the transverse plane of no greater        than 40 degrees.

Backlights for LCD panels, in their simplest form, consist of lightgeneration surfaces such as the active emitting surfaces of LED dies orthe outer layers of phosphor in a CCFL bulb, and a geometric and opticalarrangement of distributing or spreading this light in such a way as toproduce an extended- or large-area illumination surface or region,referred to as the backlight output area, which, at least in someembodiments, is spatially uniform in its emitted brightness. Generally,this process of transforming very high brightness local sources of lightinto a large-area uniform output surface results in a loss of lightbecause of interactions with the backlight cavity surfaces andinteraction with the light-generation surfaces. Other approaches such asusing direct-lit source architectures with specified LED lenses to levelthe incident first bounce flux on the front reflector can result inefficient, uniform brightness through the backlight output surface, butthese approaches are very sensitive to the exact geometricalconfiguration of all of the backlight components. To a firstapproximation, any light that is not delivered by this process throughthe output area or surface associated with a front reflector—optionallyinto a desired application viewer-cone (if any), and with a particularfiltered state (e.g., LCD-useable polarization or color)—is “lost”light. A technique for characterizing any backlight containing arecycling cavity by two essential parameters is described in PCT PatentApplication No. PCT/US2008/064096, entitled THIN HOLLOW BACKLIGHTS WITHBENEFICIAL DESIGN CHARACTERISTICS.

This characterization is particularly straightforward for planarbacklight cavities, in which the back reflector (sometimes referred toherein as a backplane) of the backlight and the output area of thebacklight are both planar, parallel to each other, of approximatelyequal area, and approximately coextensive. Our two-parametercharacterization, however, is by no means restricted to plane parallelbacklight geometries and may be generalized for any backlight geometryhaving the basic elements of an output surface associated with a frontreflector, a back reflector that forms a light recycling cavity with thefront reflector, and a grouping of one or more light sources disposedwithin, or optically connected to the cavity.

As used herein, the term “acceptable spatial uniformity” refers to bothacceptable uniformity of both overall intensity and color. What isconsidered acceptable brightness and spatial uniformity depends upon theparticular application for which the backlight will be used. Forexample, a common reference standard for LCD uniformity is TCO 05 (TheSwedish Confederation of Professional Employees, version 2.0,2005-09-21, p. 9), which specifies an acceptance threshold luminanceratio of greater than 66%. In the early commercialization of aparticular technology, uniformity standards may be lower; for example,when notebook computers were first introduced, acceptable uniformity wasin the range of 50-60%. Further, for example, internally illuminatedchannel letters are another application where luminance uniformity is animportant performance metric. Here, human factor studies have shown thatmost people judge channel letter uniformity as being acceptable if theluminance ratio is greater than 50%. See, e.g., Freyssinier et al.,Evaluation of light emitting diodes for signage applications, ThirdInternational Conference of Solid State Lighting, Proceedings of SPIE5187:309-317 (2004). Emergency signage is yet another ubiquitousapplication for light emitting panels. An example specification foruniformity is the Energy Star program for Exit Signs. See Energy StarProgram Requirements for Exit Signs Draft 1, Eligibility CriteriaVersion 3.0. For an exit sign to qualify for Energy Star designation,the sign should have a luminance nonuniformity of less than 20:1 (i.e.,greater than 5%).

One measurement for spatial uniformity that is referred to herein is theluminance and color uniformity as determined according to the VideoElectronics Standards Association's Flat Panel Display MeasurementsStandard, v. 2.0 (published Jun. 1, 2001) standard 306-1 SampledUniformity and Color of White (herein referred to as the VESA 9 ptUniformity Standard). The VESA 9 pt luminance uniformity reported hereinis determined from 9 specified circular regions referred to as “samplepoints” with locations as defined by the Standard on the output surfaceof the backlight as

${V\; E\; S\; A\mspace{14mu} 9\mspace{14mu}{pt}\mspace{14mu}{Luminance}\mspace{14mu}{Uniformity}} = \frac{L_{m\; i\; n}}{L_{m\;{ax}}}$

where L_(min) is the minimum value of the luminance of the 9 points andL_(max) is the maximum value of the luminance of the 9 points. Highervalues of VESA 9 pt luminance uniformity indicate systems that are moreuniform.

The VESA 9 pt color nonuniformity is determined as the largest value ofthe color difference between any two pairs of the 9 sampled points. Thecolor difference Δu′v′ isΔu′v′=√{square root over ((u′ ₁ −u′ ₂)²+(v′ ₁ −v′ ₂)²)}{square root over((u′ ₁ −u′ ₂)²+(v′ ₁ −v′ ₂)²)}

where the subscripts 1 and 2 denote the two regions being compared.Lower values of VESA 9 pt color nonuniformity indicate systems that aremore uniform.

As mentioned herein, the backlights of the present disclosure can beutilized as backlights for display systems. A schematic cross-sectionalview of one embodiment of a direct-lit display system 100 is illustratedin FIG. 1. Such a display system 100 may be used, for example, in an LCDmonitor or LCD-TV. The display system 100 includes a display panel 150and an illumination assembly 101 positioned to provide light to thepanel 150. The display panel 150 can include any suitable type ofdisplay. In the illustrated embodiment, the display panel 150 includesan LC panel (hereafter referred to as LC panel 150). The LC panel 150typically includes a layer of LC 152 disposed between panel plates 154.The plates 154 are often formed of glass and can include electrodestructures and alignment layers on their inner surfaces for controllingthe orientation of the liquid crystals in the LC layer 152. Theseelectrode structures are commonly arranged so as to define LC panelpixels, i.e., areas of the LC layer where the orientation of the liquidcrystals can be controlled independently of adjacent areas. A colorfilter may also be included with one or more of the plates 152 forimposing color on the image displayed by the LC panel 150.

The LC panel 150 is positioned between an upper absorbing polarizer 156and a lower absorbing polarizer 158. In the illustrated embodiment, theupper and lower absorbing polarizers 156, 158 are located outside the LCpanel 150. The absorbing polarizers 156, 158 and the LC panel 150 incombination control the transmission of light from a backlight 110through the display system 100 to the viewer. For example, the absorbingpolarizers 156, 158 may be arranged with their transmission axesperpendicular to each other. In an unactivated state, a pixel of the LClayer 152 may not change the polarization of light passing therethrough.Accordingly, light that passes through the lower absorbing polarizer 158is absorbed by the upper absorbing polarizer 156. When the pixel isactivated, the polarization of the light passing therethrough is rotatedso that at least some of the light that is transmitted through the lowerabsorbing polarizer 158 is also transmitted through the upper absorbingpolarizer 156. Selective activation of the different pixels of the LClayer 152, for example, by a controller 104, results in the lightpassing out of the display system 100 at certain desired locations, thusforming an image seen by the viewer. The controller 104 may include, forexample, a computer or a television controller that receives anddisplays television images.

One or more optional layers 157 may be provided proximate the upperabsorbing polarizer 156, for example, to provide mechanical and/orenvironmental protection to the display surface. In one exemplaryembodiment, the layer 157 may include a hardcoat over the upperabsorbing polarizer 156.

It will be appreciated that some types of LC displays may operate in amanner different from that described above. For example, the absorbingpolarizers 156, 158 may be aligned parallel and the LC panel may rotatethe polarization of the light when in an unactivated state. Regardless,the basic structure of such displays remains similar to that describedherein.

The illumination assembly 101 includes a backlight 110 and optionallyone or more light management films 140 positioned between the backlight110 and the LC panel 150. The backlight 110 can include any backlightdescribed herein, e.g., backlight 200 of FIG. 2.

An arrangement 140 of light management films, which may also be referredto as a light management unit, is positioned between the backlight 110and the LC panel 150. The light management films 140 affect theillumination light propagating from the backlight 110. For example, thearrangement 140 of light management films may include a diffuser 148.The diffuser 148 is used to diffuse the light received from thebacklight 110.

The diffuser layer 148 may be any suitable diffuser film or plate. Forexample, the diffuser layer 148 can include any suitable diffusingmaterial or materials. In some embodiments, the diffuser layer 148 mayinclude a polymeric matrix of polymethyl methacrylate (PMMA) with avariety of dispersed phases that include glass, polystyrene beads, andCaCO₃ particles. Exemplary diffusers can include 3M™ Scotchcal™ DiffuserFilm, types 3635-30, 3635-70, and 3635-100, available from 3M Company,St. Paul, Minn.

The optional light management unit 140 may also include a reflectivepolarizer 142. Any suitable type of reflective polarizer may be used forthe reflective polarizer 142, e.g., multilayer optical film (MOF)reflective polarizers; diffusely reflective polarizing film (DRPF), suchas continuous/disperse phase polarizers; wire grid reflectivepolarizers; or cholesteric reflective polarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely onthe difference in refractive index between at least two materials,usually polymeric materials, to selectively reflect light of onepolarization state while transmitting light in an orthogonalpolarization state. Some examples of MOF reflective polarizers aredescribed in co-owned U.S. Pat. No. 5,882,774 (Jonza et al.).Commercially available examples of MOF reflective polarizers includeVikuiti™ DBEF-D200 and DBEF-D440 multilayer reflective polarizers thatinclude diffusive surfaces, available from 3M Company.

Examples of DRPF useful in connection with the present disclosureinclude continuous/disperse phase reflective polarizers as described,e.g., in co-owned U.S. Pat. No. 5,825,543 (Ouderkirk et al.), anddiffusely reflecting multilayer polarizers as described, e.g., inco-owned U.S. Pat. No. 5,867,316 (Carlson et al.). Other suitable typesof DRPF are described in U.S. Pat. No. 5,751,388 (Larson).

Some examples of wire grid polarizers useful in connection with thepresent disclosure include those described, e.g., in U.S. Pat. No.6,122,103 (Perkins et al.). Wire grid polarizers are commerciallyavailable, inter alfa, from Moxtek Inc., Orem, Utah.

Some examples of cholesteric polarizers useful in connection with thepresent disclosure include those described, e.g., in U.S. Pat. No.5,793,456 (Broer et al.), and U.S. Patent Publication No. 2002/0159019(Pokorny et al.). Cholesteric polarizers are often provided along with aquarter wave retarding layer on the output side so that the lighttransmitted through the cholesteric polarizer is converted to linearlypolarized light.

In some embodiments, a polarization control layer 144 may be providedbetween the diffuser plate 148 and the reflective polarizer 142.Examples of polarization control layers 144 include a quarter waveretarding layer and a polarization rotating layer such as a liquidcrystal polarization rotating layer. The polarization control layer 144may be used to change the polarization of light that is reflected fromthe reflective polarizer 142 so that an increased fraction of therecycled light is transmitted through the reflective polarizer 142.

The optional arrangement 140 of light management films may also includeone or more brightness enhancing layers. A brightness enhancing layercan redirect off-axis light in a direction closer to the axis of thedisplay. This increases the amount of light propagating on-axis throughthe LC layer 152, thus increasing the brightness of the image seen bythe viewer. One example of a brightness enhancing layer is a prismaticbrightness enhancing layer, which has a number of prismatic ridges thatredirect the illumination light through refraction and reflection.Examples of prismatic brightness enhancing layers that may be used inthe display system 100 include the Vikuiti™ BEF II and BEF III family ofprismatic films available from 3M Company, including BEF II 90/24, BEFII 90/50, BEF IIIM 90/50, and BEF IIIT. Brightness enhancement may alsobe provided by some of the embodiments of front reflectors as is furtherdescribed herein.

The exemplary embodiment illustrated in FIG. 1 shows a first brightnessenhancing layer 146 a disposed between the reflective polarizer 142 andthe LC panel 150. A prismatic brightness enhancing layer typicallyprovides optical gain in one dimension. An optional second brightnessenhancing layer 146 b may also be included in the arrangement 140 oflight management layers, having its prismatic structure orientedorthogonally to the prismatic structure of the first brightnessenhancing layer 146 a. Such a configuration provides an increase in theoptical gain of the display system 100 in two dimensions. In otherexemplary embodiments, the brightness enhancing layers 146 a, 146 b maybe positioned between the backlight 110 and the reflective polarizer142.

The different layers in the optional light management unit 140 may befree standing. In other embodiments, two or more of the layers in thelight management unit 140 may be laminated together, for example asdiscussed in co-owned U.S. patent application Ser. No. 10/966,610 (Ko etal.). In other exemplary embodiments, the optional light management unit140 may include two subassemblies separated by a gap, for example, asdescribed in co-owned U.S. patent application Ser. No. 10/965,937(Gehlsen et al.).

The display system 100 of the embodiment illustrated in FIG. 1 caninclude any suitable backlight described herein. For example, FIG. 2 isa schematic cross-section view of one embodiment of an edge-litbacklight 200. Unless otherwise indicated, references to “backlights”are also intended to apply to other extended area lighting devices thatprovide nominally uniform illumination in their intended application.The backlight 200 includes a front reflector 210 and a back reflector220 that form a hollow light recycling cavity 202. The cavity 202includes an output surface 204. The backlight 200 also includes one ormore light sources 230 disposed to emit light into the cavity 202. Thebacklight 200 can optionally include side surfaces or reflectors 250surrounding the periphery of the light recycling cavity 200 on sidesthat do not include light sources.

As illustrated, backlight 200 includes an injector 240 that helps todirect light from the one or more light sources 230 into the lightrecycling cavity 202. Any suitable injector can be used with thebacklight 200, e.g., those injectors described in PCT Patent ApplicationNo. PCT/US2008/64125, entitled COLLIMATING LIGHT INJECTORS FOR EDGE-LITBACKLIGHTS.

Although depicted as having one or more light sources 230 positionedalong one side of the backlight 200, light sources can be positionedalong two, three, four, or more sides of the backlight 200. For example,for a rectangularly shaped backlight, one or more light sources can bepositioned along each of the four sides of the backlight.

In some embodiments, hybrid configurations are possible where lightsources are positioned both along one or more edges and also across theback reflector. In such instances it can be beneficial to position RGBlight sources along the edge and white light sources along the backreflector. White light sources can be more efficient and do not requirecolor mixing to provide white light. RGB light sources have higher colorgamut can be less efficient than white sources. This has the advantageof reducing power consumption requirements via the use of highefficiency white light sources, while increasing color gamut with theaddition of RGB sources. By injecting the RGB light along the edge,color mixing is done laterally, which can provide a thinner backlight.

The front reflector 210 is partially transmissive and partiallyreflective for at least visible light. The partial transmissivity of thefront reflector 210 allows at least a portion of light within the cavity202 to be emitted through the output surface 204 of the cavity 202. Thefront reflector 210 can include any suitable film(s) and/or layer(s)that provide partial transmission and reflection to light incident uponthe front reflector 210 from inside the cavity 202. In some embodiments,the front reflector 210 includes an on-axis average reflectivity of atleast 65%. In other embodiments, the front reflector 210 includes atotal hemispherical reflectivity of at least 75%. Still in otherembodiments, the front reflector 210 includes an on-axis averagereflectivity of at least 65% and a total hemispherical reflectivity ofat least 75%. As used herein, the term “on-axis average reflectivity”refers to the average reflectivity of light incident on a reflector in adirection that is substantially normal to such surface. Further, theterm “total hemispherical reflectivity,” i.e., R_(hemi), refers to thetotal reflectivity of a reflector for light (over a wavelength range ofinterest) incident on the reflector from all directions within ahemisphere centered around a normal to the reflector.

The front reflector 210 is operable to emit polarized light. In suchembodiments, the front reflector 210 includes an on-axis averagereflectivity of at least 90% for visible light polarized in a firstplane, and an on-axis average reflectivity of at least 25% but less than90% for visible light polarized in a second plane parallel to the firstplane. Those skilled in the art would consider light polarized in thesecond plane to be in a useable polarization state, i.e., such polarizedlight would pass through the lower absorbing polarizer of an LC panel(e.g., lower absorbing polarizer 158 of FIG. 1) and be incident on theLC panel. Further, those skilled in the art would consider the firstplane to be parallel to the block axis and the second plane to beparallel to the pass axis of the polarizing front reflector 210.Backlights of the present disclosure that provide polarized lightexhibit high enough reflectivities for useable light to providesufficient lateral transport or spreading for acceptable spatialuniformity of the emitted light, but a low enough reflectivity of usablelight to keep the overall loss of the usable polarization state in thecavity to manageable levels, thereby providing an acceptably highbrightness of the emitted light.

Further, in some embodiments, it may be desirable that the averageon-axis transmission of the useable polarization state is several timesgreater than the transmission of non-useable polarization state toensure that the output from the cavity 202 is substantially the desiredpolarization state. This also helps to reduce the total loss of useablelight from the cavity. In some embodiments, the front reflector includesa first on-axis average transmissivity for visible light polarized inthe first plane, and a second on-axis average transmissivity for visiblelight polarized in the second plane, where a ratio of the second on-axistransmissivity to the first on-axis transmissivity is at least 7. Inother embodiments, this ratio is at least 10, 20, or any suitable ratio.

The front reflector 210 can include any suitable film(s) and/or layer(s)such that the front reflector provides emitted light having the desiredoptical characteristic or characteristics. In one exemplary embodiment,the front reflector 210 can include one or more birefringent multilayeroptical films. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.)entitled OPTICAL FILM; U.S. Pat. No. 6,905,220 (Wortman et al.) entitledBACKLIGHT SYSTEM WITH MULTILAYER OPTICAL FILM REFLECTOR; U.S. Pat. No.6,210,785 (Weber et al.) entitled HIGHT EFFICIENCY OPTICAL DEVICES; andU.S. Pat. No. 6,783,349 (Neavin et al.) entitled APPARATUS FOR MAKINGMULTILAYER OPTICAL FILMS.

Multilayer optical films, i.e., films that provide desirabletransmission and/or reflection properties at least partially by anarrangement of microlayers of differing refractive index, are known. Ithas been known to make such multilayer optical films by depositing asequence of inorganic materials in optically thin layers (“microlayers”)on a substrate in a vacuum chamber. Inorganic multilayer optical filmsare described, for example, in H. A. Macleod, Thin-Film Optical Filters,2nd Ed., Macmillan Publishing Co. (1986); and A. Thelan, Design ofOptical Interference Filters, McGraw-Hill, Inc. (1989).

More recently, multilayer optical films have been demonstrated bycoextrusion of alternating polymer layers. See, e.g., U.S. Pat. No.3,610,724 (Rogers), U.S. Pat. No. 4,446,305 (Rogers et al.), U.S. Pat.No. 4,540,623 (Im et al.), U.S. Pat. No. 5,448,404 (Schrenk et al.), andU.S. Pat. No. 5,882,774 (Jonza et al.). In these polymeric multilayeroptical films, polymer materials are used predominantly or exclusivelyin the makeup of the individual layers. Such films are compatible withhigh volume manufacturing processes and can be made in large sheets androll goods.

Polymeric multilayer optical films for use in optical filters aredescribed, for example, in PCT Publication Nos. WO95/17303; WO95/17691;WO95/17692; WO95/17699; WO96/19347; and WO99/36262. One commerciallyavailable form of a multilayer reflective polarizer is marketed as DualBrightness Enhanced Film (DBEF) by 3M Company, St. Paul, Minn. Polymericmultilayer optical films are generally formed using alternating layersof polymer materials with different indices of refraction. Typically,any polymer can be used as long as the polymer is relatively transparentover the wavelength range of transmission. For polarizing applications,the first optical layers, the second optical layers, or both are formedusing polymers that are or can be made birefringent, in which thepolymer's index of refraction has differing values along orthogonalcartesian axes of the polymer. Generally, birefringent polymermicrolayers have their orthogonal Cartesian axes defined by the normalto the layer plane (z-axis), with the x-axis and y-axis laying withinthe layer plane. Birefringent polymers can also be used innon-polarizing applications.

A multilayer optical film typically includes individual microlayershaving different refractive index characteristics so that some light isreflected at interfaces between adjacent microlayers. The microlayersare sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference to givethe multilayer optical film the desired reflective or transmissiveproperties. For multilayer optical films designed to reflect light atultraviolet, visible, or near-infrared wavelengths, each microlayergenerally has an optical thickness (a physical thickness multiplied byrefractive index) of less than about 1 μm. However, thicker layers canalso be included, such as skin layers at the outer surfaces of themultilayer optical film, or protective boundary layers (PBLs) disposedbetween the multilayer optical films, that separate the coherentgroupings of microlayers. Such a multilayer optical film body can alsoinclude one or more thick adhesive layers to bond two or more sheets ofmultilayer optical film in a laminate.

In a simple embodiment, the microlayers can have thicknesses andrefractive index values corresponding to a ¼-wave stack, i.e., arrangedin optical repeat units or unit cells each having two adjacentmicrolayers of equal optical thickness (f−ratio=50%), such opticalrepeat unit being effective to reflect by constructive interferencelight whose wavelength λ is twice the overall optical thickness of theoptical repeat unit. Thickness gradients along a thickness axis of thefilm (e.g., the z-axis) can be used to provide a widened reflectionband. Thickness gradients tailored to sharpen such band edges (at thewavelength transition between high reflection and high transmission) canalso be used, as discussed in U.S. Pat. No. 6,157,490 (Wheatley et al.).For polymeric multilayer optical films, reflection bands can be designedto have sharpened band edges as well as “flat top” reflection bands, inwhich the reflection properties are essentially constant across thewavelength range of application. Other layer arrangements, such asmultilayer optical films having 2-microlayer optical repeat units whosef-ratio is different from 50%, or films whose optical repeat unitsinclude more than two microlayers, are also contemplated. Thesealternative optical repeat unit designs can be configured to reduce orto excite certain higher-order reflections. See, e.g., U.S. Pat. No.5,360,659 (Arends et al.) and U.S. Pat. No. 5,103,337 (Schrenk et al.).

Multilayer optical films can be designed to reflect one or bothpolarizations of light over at least one bandwidth. Through carefulmanipulation of these layer thicknesses and indices of refraction alongthe various film axes, the multilayer optical film can be made to behaveas a highly reflective mirror for one axis of polarization, and as aweaker, less reflective mirror for the orthogonal axis of polarization.Thus, for example, multilayer optical films may be tuned to reflectstrongly one polarization of light in the visible region of the spectrumwhile being weakly reflecting (substantially transparent) for anorthogonal polarization axis. With the appropriate choice ofbirefringence for the polymer microlayers, and the appropriate choice ofmicrolayer thicknesses, a multilayer optical film can be designed tohave any variation of reflection magnitude for polarized light alongeither of its two orthogonal in-plane axes.

Exemplary materials that can be used in the fabrication of polymericmultilayer optical film can be found in PCT Publication WO 99/36248(Neavin et al.). Exemplary two-polymer combinations that provide bothadequate refractive index differences and adequate inter-layer adhesioninclude (1) for polarizing multilayer optical film made using a processwith predominantly uniaxial stretching, PEN/coPEN, PET/coPET, PEN/sPS,PET/sPS, PEN/Eastar,™ PET/Eastar,™, PEN/FN007, where “PEN” refers topolyethylene naphthalate, “coPEN” refers to a copolymer or blend basedupon naphthalene dicarboxylic acid, “PET” refers to polyethyleneterephthalate, “coPET” refers to a copolymer or blend based uponterephthalic acid, “sPS” refers to syndiotactic polystyrene and itsderivatives, Eastar™ is a polyester or copolyester (believed to comprisecyclohexanedimethylene diol units and terephthalate units) commerciallyavailable from Eastman Chemical Co., and “FN007” (Neostar) is acopolyester ether that is commercially available from Eastman ChemicalCo.; (2) for polarizing multilayer optical film made by manipulating theprocess conditions of a biaxial stretching process, PEN/coPEN, PEN/PET,PEN/PBT, PEN/PETG and PEN/PETcoPBT, where “PBT” refers to polybutyleneterephthalate, “PETG” refers to a copolymer of PET employing a secondglycol (usually cyclohexanedimethanol), and “PETcoPBT” refers to acopolyester of terephthalic acid or an ester thereof with a mixture ofethylene glycol and 1,4-butanediol; (3) for mirror films (includingcolored minor films), PEN/PMMA, coPEN/PMMA, PET/PMMA, PEN/Ecdel,™PET/Ecdel,™ PEN/sPS, PET/sPS, PEN/coPET, PEN/PETG, and PEN/THV,™ where“PMMA” refers to polymethyl methacrylate, Ecdel™ is a thermoplasticpolyester or copolyester (believed to comprise cyclohexanedicarboxylateunits, polytetramethylene ether glycol units, and cyclohexanedimethanolunits) commercially available from Eastman Chemical Co., and THV™ is afluoropolymer commercially available from 3M Company.

Further details of suitable multilayer optical films and related designsand constructions can be found in U.S. Pat. No. 5,882,774 (Jonza etal.), U.S. Pat. No. 6,531,230 (Weber et al.), PCT Publication Nos. WO95/17303 (Ouderkirk et al.), WO 99/39224 (Ouderkirk et al.), and “GiantBirefringent Optics in Multilayer Polymer Minors”, Science, Vol. 287,March 2000 (Weber et al.).

Multilayer optical films and film bodies can include additional layersand coatings selected for their optical, mechanical, and/or chemicalproperties. For example, a UV absorbing layer can be added at theincident side of the optical element to protect components fromdegradation caused by UV light. Additional layers and coatings couldalso include scratch resistant layers, tear resistant layers, andstiffening agents. See, e.g., U.S. Pat. No. 6,368,699 (Gilbert et al.).

FIG. 3 depicts a conventional multilayer optical film 300. The film 300includes individual microlayers 302, 304. The microlayers have differentrefractive index characteristics so that some light is reflected atinterfaces between adjacent microlayers. The microlayers aresufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference to givethe film the desired reflective or transmissive properties. For opticalfilms designed to reflect light at ultraviolet, visible, ornear-infrared wavelengths, each microlayer generally has an opticalthickness (i.e., a physical thickness multiplied by refractive index) ofless than about 1 μm. Thicker layers can, however, also be included,such as skin layers at the outer surfaces of the film, or protectiveboundary layers disposed within the film that separate packets ofmicrolayers.

The reflective and transmissive properties of multilayer optical film300 are a function of the refractive indices of the respectivemicrolayers. Each microlayer can be characterized, at least in localizedpositions in the film, by in-plane refractive indices n_(x), n_(y), anda refractive index n_(z) associated with a thickness axis of the film.These indices represent the refractive index of the subject material forlight polarized along mutually orthogonal x-, y-, and z-axes,respectively (see FIG. 3).

In practice, the refractive indices are controlled by judiciousmaterials selection and processing conditions. Film 300 can be made byco-extrusion of typically tens or hundreds of layers of two alternatingpolymers A, B, followed by optionally passing the multilayer extrudatethrough one or more multiplication die, and then stretching or otherwiseorienting the extrudate to form a final film. The resulting film iscomposed of typically tens or hundreds of individual microlayers whosethicknesses and refractive indices are tailored to provide one or morereflection bands in desired region(s) of the spectrum, such as in thevisible or near infrared. To achieve high reflectivities with areasonable number of layers, adjacent microlayers can exhibit adifference in refractive index (Δn_(x)) for light polarized along thex-axis of at least 0.05. If the high reflectivity is desired for twoorthogonal polarizations, then the adjacent microlayers also can exhibita difference in refractive index (Δn_(y)) for light polarized along they-axis of at least 0.05.

If desired, the refractive index difference (Δn_(z)) between adjacentmicrolayers for light polarized along the z-axis can also be tailored toachieve desirable reflectivity properties for the p-polarizationcomponent of obliquely incident light. For ease of explanation, at anypoint of interest on a multilayer optical film, the x-axis will beconsidered to be oriented within the plane of the film such that themagnitude of Δn_(x) is a maximum. Hence, the magnitude of Δn_(y) can beequal to or less than (but not greater than) the magnitude of Δn_(x).Furthermore, the selection of which material layer to begin with incalculating the differences Δn_(x), Δn_(y), Δn_(z) is dictated byrequiring that Δn_(x) be non-negative. In other words, the refractiveindex differences between two layers forming an interface areΔn_(j)=n_(1j)−n_(2J), where j=x, y, or z and where the layerdesignations 1, 2 are chosen so that n_(1x)≧n_(2x)., i.e., Δn_(x)≧0.

To maintain high reflectivity of p-polarized light at oblique angles ofincidence, the z-index mismatch Δn_(z) between microlayers can becontrolled to be substantially less than the maximum in-plane refractiveindex difference Δn_(x), such that Δn_(z)≦0.5*Δn_(x). More preferably,Δn_(z)≦0.25*Δn_(x). A zero or near zero magnitude z-index mismatchyields interfaces between microlayers whose reflectivity for p-polarizedlight is constant or near constant as a function of incidence angle.Furthermore, the z-index mismatch Δn_(z) can be controlled to have theopposite polarity compared to the in-plane index difference Δn_(x),i.e., Δn_(z)<0. This condition yields interfaces whose reflectivity forp-polarized light increases with increasing angles of incidence, as isthe case for s-polarized light.

Alternatively, the multilayer optical film can have a simplerconstruction in which all of the polymeric microlayers are isotropic innature, i.e., n_(x)=n_(y)=n_(z) for each layer. Furthermore, knownself-assembled periodic structures, such as cholesteric reflectingpolarizers and certain block copolymers, can be considered multilayeroptical films for purposes of this application. Cholesteric mirrors canbe made using a combination of left- and right-handed chiral pitchelements.

In reference to traditional polarizing films, light can be considered tobe polarized in two orthogonal planes, where the electric vector of thelight, which is transverse to the propagation of the light, lies withina particular plane of polarization. In turn, the polarization state of agiven light ray can be resolved into two different polarization states:p-polarized and s-polarized light. P-pol light is light that ispolarized in the plane of incidence of the light ray and a givensurface, where the plane of incidence is a plane containing both thelocal surface normal vector and the light ray propagation direction orvector.

For example, FIG. 4 illustrates light ray 410 that is incident on apolarizer 402 at an angle of incidence θ, thereby forming a plane ofincidence 412. The polarizer 402 includes a pass axis 404 that isparallel to the y-axis, and a block axis 406 that is parallel to thex-axis. The plane of incidence 412 of ray 410 is parallel to the blockaxis 406. Ray 410 has a p-polarized component that is in the plane ofincidence 412, and an s-polarized component that is orthogonal to theplane of incidence 412. The p-pol light of ray 410 is parallel to theblock axis 406 of polarizer 402 and will, therefore, be substantiallyreflected by the polarizer, while the s-pol light of ray 410 is parallelto the pass axis 404 of polarizer 402 and, at least in part, betransmitted.

Further, FIG. 4 illustrates ray 420 that is incident on polarizer 402 ina plane of incidence 422 that is parallel to the pass axis 404 of thepolarizer 402. Therefore, the p-pol light of ray 420 is parallel to thepass axis 404 of the polarizer 402, while the s-pol light of ray 420 isparallel to the block axis 406 of polarizer 402 As a result, assumingthat the polarizer 402 is a perfect polarizer that has a reflectance of100% at all angles of incident light for light polarized in the blockaxis and 0% at all angles of incident light for light polarized in thepass axis, the polarizer transmits s-pol light of ray 410 and the p-pollight of ray 420, while reflecting the p-pol light of ray 410 and thes-pol light of ray 420. In other words, the polarizer 402 will transmita combination of p- and s-pol light. The amount of transmission andreflection of p- and s-pol light will depend on the characteristics ofthe polarizer as is further described herein.

In general, various asymmetric reflective films can be provided for useas a front reflector (e.g., front reflector 210 of FIG. 2) by alteringthe relative degree of index match of an in-plane index of the low indexmaterial with the z-index of the adjacent birefringent high indexmaterial. In some embodiments, relatively large in-plane indexmismatches are required along both in-plane optical axes of theasymmetric reflective film, but the mismatches are significantlydifferent from each other, thus producing asymmetrical normal incidencetransmission and reflection properties. This is in contrast toconventional reflective polarizing films where in-plane indices aresubstantially matched along the pass axis. An example of such films isDBEF (available from 3M Company), which has low reflectivity for lightpolarized along one in-plane axis at normal incidence.

For example, an exemplary asymmetric reflective film that can be used inthe front reflector of the present disclosure can have a high indexlayer (i.e., the layer that includes the highest index of refraction)with in plane index values of nx1=1.82 and ny1=1.62, and a z-axis indexof nz1=1.50, and an isotropic low index layer having in-plane indices ofnx2=ny2=nz2=1.56. A film having these indices of refraction can beformed using a coPEN/PETG coextruded multilayer film using a constraineduniaxial orientation as in a standard film tenter. Using about 300layers, the reflectivities shown in FIG. 5 can be achieved for lightfrom 400 to 870 nm with polarization vectors parallel to the y-z plane(the “pass” axis). Due to the large index difference along the x-axis,and the lack of a Brewster angle, about 98% of light with polarizationvectors parallel to the x-z plane is reflected. FIG. 5 illustrates thereflectivity of light for the pass axis at various angles of incidencein air for p-pol light (curve 502) and s-pol light (curve 504). Asillustrated, such a film can include an average on-axis reflectivity ofabout 29% for visible light for one polarization while having a muchhigher reflectivity of about 98% for the block axis.

In general, the use of a high index biaxially birefringent material,such as the one illustrated in FIG. 5, allows for the design ofasymmetric reflectors which block most light components polarizedparallel to a first (block) axis, and pass controlled amounts of boths-polarized and p-polarized light components that are aligned with theorthogonal (pass) axis. The relative reflectivity for s- and p-pol lightalong this pass axis can be adjusted by varying the isotropic index n2of the second material to a value somewhere between ny1 and nz1.

The asymmetric reflective film or films utilized for the front reflectorcan include a high index material that is highly biaxially birefringent,having indices of n_(x1)>>n_(y1)>>n_(z1). This can be achieved via aconstrained uniaxial stretch of some materials, or an asymmetricalorientation of these or other materials. This relationship enables thedesign of a film that simultaneously meets the following criteria:

-   -   The value of Δn_(y) is large enough so that a substantial        reflectivity (e.g., 25% to 90%) can be achieved for the pass        axis with a useful number of layers. This constraint relates to        spectral control needed for low color films as is further        described herein. In some embodiments, it may be preferred that        Δn_(y)≧about 0.05.    -   The value of Δn_(x) can be significantly larger than Δn_(y) to        insure that the block axis transmits much less light than the        pass axis. In general, it may be preferred that Δn_(x)≧2Δn_(y).    -   Δn_(z) can be much less than Δn_(x) and, in some embodiments, is        of the opposite sign. This can help to prevent light polarized        along the block axis from leaking, especially at oblique        incidence angles.

Front reflectors that provide a polarized output and reflect at leastsome light in the pass state can provide acceptable spatial uniformityof emitted light from backlights that include one or more light sourcesby increased recycling of the light within the light recycling cavity. Aschematic representation of a portion of this type of backlight isillustrated in FIG. 6, where backlight 600 includes a front reflector610 and a back reflector 620 that form a hollow light recycling cavity602. Light 660 within the cavity 602 represents light that is incidenton the front reflector 610 and includes light of a first polarizationstate (a) and a second orthogonal polarization state (b). The frontreflector 610 transmits a portion of light 662 having polarization state(a) while reflecting a second portion 664 of light with state (a) andreflecting substantially all of light 666 having polarization state (b).The reflected light 664, 666 is reflected by back reflector 620 anddirected toward front reflector 610 where again a portion 668 of state(a) is transmitted, and a second portion of state (a) and substantiallyall of state (b) is reflected. As a result of this reflection of lightof both polarization states (a) and (b), the light within the cavity 602is allowed to travel laterally in the cavity in direction 670. In atypical backlight, light of polarization state (a) may be substantiallytransmitted by the backlight on the first pass, thereby reducing theamount of light that is transported laterally within the cavity.Although depicted in two dimensions, it is understood that thebacklights of the present disclosure can provide lateral transport oflight in both orthogonal directions within the cavity such that lightsubstantially fills the cavity to provide acceptable spatial uniformityof the light emitted from the cavity.

The front reflectors of the present disclosure not only help to provideacceptable spatial uniformity, some embodiments of front reflectors alsoprovide angularly selective transmission of useable light, e.g., to adisplay.

In general, at least some embodiments of the front reflectors exhibit areflectivity that generally increases with angle of incidence of light,and a transmission that generally decreases with angle of incidence,where the reflectivity and transmission are for unpolarized visiblelight and for any plane of incidence, and/or for light of a usablepolarization state incident in a plane for which oblique light of theusable polarization state is p-polarized. For example, FIGS. 5 and 7A-Billustrate reflectivity in the pass-axis versus angle of incidence forvarious embodiments of front reflectors of the present disclosure.

FIG. 5 illustrates the pass axis reflectivity versus incident angle forvisible light in air for one embodiment of a front reflector as modeledusing standard modeling techniques. A front reflector having thereflectivities shown in FIG. 5 can be formed using a coPEN/PETGcoextruded multilayer film using a constrained uniaxial orientation asin a standard film tenter. Using about 300 layers, the reflectivitiesshown in FIG. 5 can be achieved for visible light from 400 to 870 nmwith polarization vectors parallel to the y-z plane (i.e., the passaxis).

Curve 502 represents the reflectivity of p-pol light in the pass axisand curve 504 represents the reflectivity of s-pol light in the passaxis. The reflectivity values include the reflections from themultilayer film and surface reflections at the air/film boundary. As canbe seen in FIG. 5, the reflectivity for both s-pol and p-pol lightincreases with increasing angle of incidence. Due to the large indexdifference along the x-axis, and the lack of a Brewster angle, about 98%of light with polarization vectors parallel to the x-z plane isreflected (i.e., the block axis). This single film can thus perform thetask of multiple films to form a front reflector that transmitscontrolled amounts of light polarized parallel to a pass axis. Thecalculated average reflectivity for s- and p-polarized light at 60degrees for the pass axis is about 50%. Further, the reflectivity forall light polarized in a plane parallel to the block axis can be greaterthan about 99%.

In general, the use of a high index biaxially birefringent material,such as the asymmetric reflective film described regarding FIG. 5,allows for design of asymmetric reflectors that block most lightcomponents polarized parallel to one axis, and pass controlled amountsof both s-polarized and p-polarized light components that are alignedwith the orthogonal (pass) axis. The relative reflectivity for s- andp-pol light along this pass axis can be adjusted by varying theisotropic refractive index n2 of the second material to a valuesomewhere between ny1 and nz1.

The index relationships, their numerical values, and the multilayerstack design should all be considered to create a front reflector that,in some embodiments, has an intermediate transmission value along oneaxis (i.e., the pass axis) and about ≦10× that value of transmissionalong the other (block) axis at normal incidence. In addition, it may bepreferred that the block axis does not leak much light at any angle ofincidence and, therefore, such embodiments require either a largeBrewster angle or no Brewster angle for the block axis. The pass axiscan have a Brewster angle if the system in which it is employed canaccept an angular distribution of light that is shifted towards offnormal angles.

The “high” index material, defined as the material with the highestin-plane index, can be highly biaxially birefringent in order thatn_(x1)>>n_(y1)>>n_(z1). This can be achieved via a constrained uniaxialstretch of some materials, or an asymmetrical orientation of these orother materials. This relationship enables the design of a film thatsimultaneously meets the following three criteria:

-   -   1) For reflectivities of at least 25% but less than 90% for        visible light polarized in the pass axis, the value of Δn_(y)        should be large enough so that such substantial reflectivity can        be achieved for the pass axis with a useful number of layers.        With practical materials and process systems this leads to the        requirement that Δn_(y)≧about 0.05.    -   2) To help mitigate transmission of light in the block axis, the        value of Δn_(x) should be significantly larger than Δn_(y). In        general, it may be preferred that Δn_(x)≧2*Δn_(y). Further, it        may be preferred that Δn_(z) is much less than Δn_(x) and of the        opposite sign.

As stated herein, the front reflector can provide increasingreflectivity for both s-pol and p-pol light as a function of angle ofincidence. This effect will produce an angular “gain” in a backlightdisplay, similar to the effect of prismatic BEF films, or gain diffuserfilms commonly used in the industry. With the front reflectors, the highreflectivity at oblique angles recycles oblique rays which are convertedin the backlight to low angle rays that have a higher probability oftransmission. In this manner, more light exits the light recyclingcavity near normal incidence than at high angles.

In typical LCD panels, the pass axis of the lower absorbing polarizer(i.e., lower absorbing polarizer 156 of FIG. 1) is often placedhorizontally on the LCD panel. With this arrangement, p-pol light isincident in the horizontal plane and s-pol light is incident in thevertical plane. The lateral viewing angle for such panels is typicallydesired to be much wider than the vertical viewing angle, although itmay be desirable to control those relative values. Therefore, therelative amount of s-pol vs. p-pol reflectivity and subsequent angulargain is a parameter that is desirably controlled. In general, the passaxis of the lower absorbing polarizer can be placed in any suitableorientation relative to the viewer, e.g., at any angle to horizontal,vertically, etc.

FIG. 7A illustrates the pass axis reflectivity versus incident angle forlight in air for another embodiment of a front reflector as modeledusing standard modeling techniques. A front reflector having thereflectivities shown in FIG. 7A can be formed using an coPEN/PMMAcoextruded multilayer film using a constrained uniaxial orientation asin a standard film tenter. Using about 300 layers, the reflectivitiesshown in FIG. 7A can be achieved for light from 400 to 870 nm withpolarization vectors parallel to the y-z plane (i.e., the pass axis).The refractive indices for the microlayers of coPEN are nx1=1.82,ny1=1.61, and nz1=1.52. And the indices for the microlayers of PMMA arenx2=ny2=nz2=1.49. Curve 706 represents the reflectivity of p-pol lightin the pass axis and curve 708 represents the reflectivity of s-pollight in the pass axis. Curve 710 represents the reflectivity of both p-and s-pol light in the block axis. The reflectivity values include thereflections from the multilayer film and surface reflections at theair/film boundary. The materials and indices of refraction of thisembodiment can be used in a multilayer stack design to create angulargain or collimation for s-pol light but not for p-pol light that istransmitted by the pass axis of this front reflector.

Index n2 is slightly less than nz1, and the reflectivity of the passaxis for p-pol light decreases with increasing angle of incidence, asillustrated in FIG. 7A. With a film such as this in the LCD panel with ahorizontal pass axis, more light would exit the cavity to either sidethan at normal incidence, i.e., the panel would be brighter when viewedfrom the side than at normal incidence. The addition of gain diffuserfilms or a BEF film with vertically oriented grooves could be used todirect more light to the normal (perpendicular) viewing direction.Alternatively, the low index material can be changed so that it has anindex above that of nz1. Light exiting the cavity in the vertical planeis predominantly s-polarized. As shown in FIG. 7A, the reflectivity fors-pol light increases substantially as a function of angle of incidence.This front reflector would thus recycle much of the high angle lightincident in the vertical plane, thereby creating substantial angulargain in the vertical direction.

In summary, referring to FIGS. 5 and 7A, by selecting the refractiveindex value of the low index material, in a range between the high indexmaterial values of ny1 and nz1, the relative strengths of s-pol andp-pol reflectivity can be controlled as a function of angle. In thismanner, the angular gain or collimation of polarized light from thebacklight cavity in each direction can be controlled. Index ny2 can bemade lower than nz1, but it may be desirable to not make ny2 so low thatthe Brewster angle thus created along the pass axis will leak p-pollight incident at high angles of incidence. Such a design will create anegative angular gain. Making ny2 larger than ny1 can cause the sameresult. Additionally, making ny2 larger than ny1 will lower thereflectivity for the block axis of the reflector, thereby potentiallyrequiring additional layers for the film to prevent leakage of lighthaving an undesirable polarization state.

The relative amounts of s-pol vs. p-pol angular gain can also beadjusted if both the high index and low index materials arebirefringent. Such materials for the low index layer can be selected tohave the same or opposite birefringence as the high index layer, and therelative values of ny2 and nz2 can be selected to determine the relativemagnitudes of s-pol and p-pol reflectivity.

For example, FIG. 7B illustrates the pass axis reflectivity versusincident angle for light in air for another embodiment of a frontreflector as modeled using standard modeling techniques. A frontreflector having the reflectivities shown in FIG. 7B can be formed usinga coPEN high index material and a low index material syndiotacticpolystyrene (sPS) or polyvinyl naphthalene (PVN) or other suitablematerial. The materials can be formed into a coextruded multilayer filmusing a constrained uniaxial orientation as in a standard film tenter.Using about 275 layers, the reflectivities shown in FIG. 7B can beachieved for light from 400 to 870 nm with polarization vectors parallelto the y-z plane (i.e., the pass axis). The refractive indices for themicrolayers of coPEN are nx1=1.82, ny1=1.61, and nz1=1.50. And theindices for the microlayers of sPS (or other suitable material) arenx2=1.52, ny2=1.57, and nz2=1.65. Curve 712 represents the reflectivityof p-pol light in the pass axis and curve 714 represents thereflectivity of s-pol light in the pass axis. The reflectivity valuesinclude the reflections from the multilayer film and surface reflectionsat the air/film boundary.

In general, when both materials are birefringent, if the low indexmaterial has the opposite birefringence to the high index material, thenΔnz can be increased while decreasing, or maintaining, the value of Δny.If the low index material has the same sign of birefringence as the highindex material, then Δnz can be decreased while maintaining ordecreasing the value of Δny. As illustrated in FIG. 7B, the p-pol light(curve 712) increases as a function of incidence angle at a greater ratethan does the s-pol light (curve 714). As a result, the p-pol lightexhibits a much greater angular gain or collimation than does thes-polarized light.

In general, prismatic brightness enhancing films act as partialcollimators of light by reflecting on-axis light and refracting off-axislight. For example, FIG. 28 is a schematic cross-section view of aportion of a prismatic brightness enhancing film 2800. Film 2800 has asmooth side 2802 and a structured side 2804. Structured side 2804includes a plurality of triangular prisms 2806. Light ray 2810 isincident upon smooth surface 2802 at a grazing angle, i.e., an angle tothe normal approaching 90 degrees, and is refracted. Upon reachingstructured surface 2804, ray 2810 is again refracted. Light ray 2812approaches smooth surface 2802 at an angle much closer to the normal tosmooth surface 2802 than ray 2810. It also is refracted as it passesthrough both the smooth surface 2802 and the structured surface 2804.Further, light ray 2814 is incident upon smooth surface 2802 at an angleeven closer to the normal to smooth surface 2802 than was light ray2838, and is totally internally reflected twice by structured surface2804.

As illustrated, light rays that are incident upon the brightnessenhancing film 2800 at relatively high angles (i.e., ray 2810) tend tobe refracted by the prismatic surfaces towards the normal, while lightrays that are incident at relatively low angles (i.e., ray 2814) tend tobe reflected (by TIR at the prism surfaces) back towards the incidentdirection. By this process, light rays from an angle-mixed source, suchas a recycling cavity, are concentrated through the structured surface2804 towards the normal angle. Light that is reflected back into thecavity from the TIR process at the prism faces can be reflected by aback reflector in a typical light recycling cavity. If the backreflector is at least partially diffusely reflective, then thatreflected light is again angle mixed, and the recycling process can leadto an increase in brightness about the normal angle viewer cone,compared to the brightness into the viewer cone without the brightnessenhancing film 2800.

The angular performance of various front reflectors can be measuredusing a gain cube. See, e.g., U.S. Patent Publication No. 2006/0082700(Gehlsen et al.), entitled COMPOSITE DIFFUSER PLATES AND DIRECT-LITLIQUID CRYSTAL DISPLAYS USING SAME. The gain cube used to measure thefollowing front reflector embodiments included a 5″ Teflon® cube that ismade of Teflon® (PTFE) walls that were ⅝″ thick on the side and ¼″ thickon the top. The cube had a Teflon® bottom with an aluminum bottom plate.An LED on a circuit board was mounted on this plate using thermal tape.2×TIPS (see Examples for description) was used to line the bottom withholes cut out for the LEDs. This recycling cavity configuration resultedin a spatially uniform (across the backplane) diffuse illumination, anda moderately reflective, highly diffuse recycling cavity, therebyproviding a simple recycling cavity device that can be used to measurebrightness change for a recycling backlight cavity for various frontreflector embodiments.

For the following embodiments, a Sanritz, model HLC2-5618S absorbingpolarizer was placed atop the gain cube and conoscopic brightness wasmeasured using a Conoscope™ optical measurement system, available fromautronic-MELCHERS GmbH, Karlsruhe, Germany. Baseline measurements wereperformed by placing the absorbing polarizer on top of the gain cube.The various front reflectors were then placed on top of the gain cube,and the absorbing polarizer was placed on top of the front reflector. Ameasurement of conoscopic brightness made in this configuration thendemonstrated the change in simple recycling backlight brightness as afunction of viewer (measurement) observation angle. Measured brightnessvalues are shown for various front reflector embodiments, where thebrightness is plotted for a range of angles from normal to grazing, forazimuth angle of 0° and 90°. For 0° azimuth angle, light that is alignedwith the absorbing polarizer pass axis is s-polarized, and for a 90°azimuth angle, light that is aligned with the absorbing polarizer passaxis is p-polarized.

FIG. 29 is a graph of luminance versus polar angle for a single sheet ofBEF. Curves 2902 and 2904 represent BEF at 0° and 90° respectively, andcurves 2906 and 2908 represent the absorbing polarizer at 0° and 90°respectively without the BEF. The brightness to the normal angle conewas increased by a factor of 1.605, with brightness enhancementextending to a wide range of angles along the 90° azimuth plane (in thedirection of the grooves), and to a narrower range of angles along the0° azimuth plane (in the direction perpendicular to the grooves). Inaddition, the brightness at high angles was significantly diminishedrelative to the output of the gain cube having only the absorbingpolarizer on the output surface.

FIG. 30 is a graph of luminance versus polar angle for two sheets of BEFthat are crossed. Curves 3002 and 3004 represent the crossed BEF at 0°and 90° respectively, and curves 3006 and 3008 represent the absorbingpolarizer at 0° and 90° respectively without the crossed BEF. Thebrightness to the normal angle cone was increased by a factor of 2.6,with brightness enhancement significantly narrowing along both the 90°azimuth plane and the 0° azimuth plane.

FIG. 31 is a graph of luminance versus polar angle for APF (as furtherdescribed in the Examples) with its pass axis aligned with the pass axisof the overlaying absorbing polarizer. Curves 3102 and 3104 representthe APF at 0° and 90° respectively, and curves 3106 and 3108 representthe absorbing polarizer at 0° and 90° respectively without the APF. TheAPF front reflector increases the brightness to the normal angle cone bya factor 1.72, with brightness enhancement remaining very wide to highangles for the 90° azimuth plane, and narrowing along the 0° azimuth.

FIG. 32 is a graph of luminance versus polar angle for DBEF with itspass axis aligned with the pass axis of the overlaying absorbingpolarizer. Curves 3202 and 3204 represent DBEF at 0° and 90°respectively, and curves 3206 and 3208 represent the absorbing polarizerat 0° and 90° respectively without the DBEF. The DBEF front reflectorincreased the brightness to the normal angle cone by a factor 1.66, andas with the APF front reflector, the brightness enhancement remainedvery wide to high angles for the 90° azimuth plane, and narrowing alongthe 0° azimuth.

FIG. 33 is a graph of luminance versus polar angle for 3×ARF (seeExamples for description) with its pass axis aligned with the pass axisof the overlaying absorbing polarizer. Curves 3302 and 3304 represent3×ARF at 0° and 90° respectively, and curves 3306 and 3308 represent theabsorbing polarizer at 0° and 90° respectively without 3×ARF. The 3×ARFfront reflector increases the brightness to the normal angle cone by afactor 1.84, with brightness enhancement narrowing at higher angles forboth the 90°azimuth plane and the 0° azimuth plane. This increase in thenormal angle brightness is well above the standard reflective polarizerfilms APF and DBEF, and may be caused by the increased hemisphericalreflectivity of the 3×ARF front reflector, as compared with that for APFand DBEF (see Table 1). This normal angle viewer cone brightnessincrease occurs even with an increase in the on-axis transmission, asthe added recycling from the 3×ARF film causes the recycling and anglemixed cavity light rays to have a higher probability of transmittingthrough the front reflector. In addition, the reduction in high anglebrightness through the output surface can be advantageous as additionalprismatic or refractive components may not be necessary to keep the LCDpanel contrast ratio at required levels.

TABLE 1 Hemispherical On-Axis Front reflector Reflectivity TransmissionAPF 51.0% 89.3% DBEF 50.8% 87.5% 3xARF 75.4% 52.0% Bead-coated ARF-8692.1% 12.8%

FIG. 34 is a graph of luminance versus polar angle for ARF-86 (sameconstruction as bead-coated ARF-84. See Examples for description) withits pass axis aligned with the pass axis of the overlaying absorbingpolarizer. Curves 3402 and 3404 represent ARF-86 at 0° and 90°respectively, and curves 3406 and 3408 represent the absorbing polarizerat 0° and 90° respectively without ARF-86. In FIG. 34 the brightnessincrease is essentially zero in the normal angle cone, because the frontreflector is highly reflecting for light polarized along the pass axisof both the ARF-86 and the absorbing polarizer. Even with ahemispherical reflectivity of 92.1%, the brightness in the normal angleviewer cone is unchanged relative to the absorbing polarizer alone.

FIG. 35 is a graph of luminance versus polar angle for ARF-86 (seeExamples for description) having a bead-coated top surface, with itspass axis aligned with the pass axis of the overlaying absorbingpolarizer. Curves 3502 and 3504 represent ARF-86 at 0° and 90°respectively, and curves 3506 and 3508 represent the absorbing polarizerat 0° and 90° respectively without ARF-86. In this instance, thebead-coated diffusing surface is on a surface of the ARF-86 opposite thegain cube cavity. As such, it has a tendency to collimate angle mixedlight that encounters the surface structure from below throughrefractive effects. At the same time, the bead-coated surface tends toimpart a degree of polarization randomization to the polarized lightrays that emerge from the ARF-86 film, and up through the bead-coatedsurface as they exit the cavity through the output surface. This effectcauses the p-polarized brightness to decrease with increasing angle,compared to ARF-86 without the bead coating.

Returning to FIG. 2, the front reflector 210 can include one or morefilms or layers that provide the desired reflectivity and transmissioncharacteristics. In some embodiments, the front reflector can includetwo or more films. For example, FIG. 8A is a schematic cross-sectionview of a portion of a front reflector 800. Reflector 800 includes afirst film 802 positioned proximate a second film 804. The films 802,804 can be spaced apart or in contact with each other. Alternatively,the films 802, 804 can be attached using any suitable technique. Forexample, the films 802, 804 can be laminated together using optionaladhesive layer 806. Any suitable adhesive can be used for layer 806,e.g., pressure sensitive adhesives (such as 3M Optically ClearAdhesives), and UV-curable adhesives (such as UVX-4856). In someembodiments, adhesive layer 806 can be replaced with an index matchingfluid, and the films 802, 804 can be held in contact using any suitabletechniques known in the art.

Films 802, 804 can include any suitable films described herein in regardto the front reflector. Films 802, 804 can have similar opticalcharacteristics; alternatively, films 802, 804 can be differentconstructions that provide different optical characteristics. In oneexemplary embodiment, film 802 can include an asymmetric reflective filmas described herein having a pass axis in one plane, and film 804 caninclude a second asymmetric reflective film having a pass axis in asecond plane that is non-parallel to the pass axis of the first film802. This non-parallel relationship can form any suitable angle betweenthe two pass axis planes. In some embodiments, the pass axis planes canbe nearly orthogonal. Such a relationship would provide a high degree ofreflectivity in the pass axis for the front reflector 800.

Further, for example, film 802 may include an asymmetric reflectivefilm, and film 804 may include a prismatic brightness enhancing filmsuch as BEF. In some embodiments, the BEF may be oriented in relation tothe asymmetric reflective film such that the BEF collimates transmittedlight in a plane that is orthogonal to the collimating plane of theasymmetric film. Alternatively, in other embodiments, the BEF may beoriented such that the BEF collimates transmitted light in thecollimating plane of the asymmetric reflective film.

In other embodiments, film 802 can include any suitable film describedherein, e.g., an asymmetric reflective film, and film 804 can be anysuitable substrate layer. The substrate can include any suitablematerial or materials, e.g., polycarbonate; acrylic; PET; fiberreinforced optical film as described, e.g., in U.S. Patent PublicationNo. 2006/0257678 (Benson et al.), entitled FIBER REINFORCED OPTICALFILMS; U.S. patent application Ser. No. 11/323,726 (Wright et al.),entitled REINFORCED REFLECTIVE POLARIZER FILMS; and U.S. patentapplication Ser. No. 11/322,324 (Ouderkirk et al.), entitled REINFORCEDREFLECTIVE POLARIZER FILMS.

Although the front reflector 800 is depicted as including two films 802,804, the front reflector 800 can include three or more films. Forexample, a three layer front reflector can be made using three layers ofreflective polarizer (such as DBEF or APF). If the three layers arearranged such that the polarization axis of the second layer is at 45°relative to the polarization axis of the first layer and thepolarization axis of the third layer is at 90° relative to thepolarization axis of the first layer, the resulting front reflector willreflect approximately 75% of the normal incidence light. Other angles ofrotation between the layers could be used to achieve different levels ofreflection.

A birefringent (polarization rotating) layer or a scattering layerbetween two reflective polarizers with nearly orthogonal pass axes canalso create reflective films that have a controlled degree ofreflectivity to be used as the front reflector. The pass axes of the tworeflective polarizers may be aligned, biased, or orthogonal. The layerbetween the reflective polarizers may be a birefringent plate, prismaticfilm, diffuser, or other optical films having the property of rotatingor scrambling the polarization passing through the first reflectivepolarizer. The layer between the reflective polarizers may also be acombination of two or more films, such as a support plate and adiffuser. The layers may be unattached to each other, or may be attachedby lamination or other attachment processes.

The front reflectors of the present disclosure can also include opticalelements positioned in or on one or more surfaces of the reflector. Forexample, FIG. 8B is a schematic cross-section view of a portion ofanother embodiment of front reflector 810. The reflector 810 includes afilm 812 having a first major surface 814 and a second, major surface816. The film 812 can include any suitable film(s) or layer(s) describedherein in regard to a front reflector. A plurality of optical elements818 are positioned on or in the first major surface 814. Althoughdepicted as positioned only on first major surface 814, optical elementscan be positioned on the second major surface 816 or on both first andsecond major surfaces 814, 816. Any suitable optical elements can bepositioned on or in the film 812, e.g., microspheres, prisms,cube-corners, lenses, etc. The optical elements can be refractiveelements, diffractive elements, diffusive elements, etc. In thisembodiment, the optical elements 818 can collimate light that istransmitted by film 812. In other embodiments, the optical elements 818can diffuse light either incident on the film 812 or exiting the film812, depending upon the positioning of the optical elements 812.

The optical elements 818 can be positioned on a major surface of thefilm 812 or at least partially embedded in the major surface of the film812. Further, the film 810 can be manufactured using any suitabletechnique, e.g., those techniques described herein for manufacturingbead-coated ESR.

The optical elements 818 can also be positioned on a cover layer orsubstrate that is positioned proximate the film 810. For example, FIG.8C is a schematic cross-section view of a portion of another embodimentof a front reflector 820. The reflector 820 includes a film 822 and again diffuser 824 positioned proximate the film 822. The film 820 caninclude any film(s) and/or layer(s) described herein regarding frontreflectors. The gain diffuser 824 includes a substrate 826 having afirst major surface 828 and a second major surface 830, and a pluralityof optical elements 832 positioned on or in the second major surface 830of the substrate 826. Any suitable optical elements 832 can be used,e.g., optical elements 818 of FIG. 8B. The substrate 826 can include anysuitable optically transmissive substrate.

For the embodiment illustrated in FIG. 8C, the first major surface 828of the gain diffuser 824 is positioned proximate the film 822. Thediffuser 824 can be positioned proximate film 822 such that it is spacedapart from the film 822, in contact with the film 822, or attached tothe film 822. Any suitable technique can be used to attach the diffuser824 to the film 822, e.g., the use of optical adhesives. Any suitablegain diffuser can be used for diffuser 824. In some embodiments, theoptical elements 832 can be positioned on the first major surface 828 ofthe substrate 826 such that the elements 832 are between the substrate826 and the polarizing film 822.

Returning to FIG. 2, the front reflector 210 can also be attached to asupporting layer. The support layer can include any suitable material ormaterials, e.g., polycarbonate, acrylic, PET, etc. In some embodiments,the front reflector 210 can be supported by a fiber reinforced opticalfilm as described, e.g., in U.S. Patent Publication No. 2006/0257678(Benson et al.), entitled FIBER REINFORCED OPTICAL FILMS; U.S. patentapplication Ser. No. 11/323,726 (Wright et al.), entitled REINFORCEDREFLECTIVE POLARIZER FILMS; and U.S. patent application Ser. No.11/322,324 (Ouderkirk et al.), entitled REINFORCED REFLECTIVE POLARIZERFILMS. Further, the front reflector 210 can be attached to the supportlayer using any suitable technique. In some embodiments, the frontreflector 210 can be adhered to the support layer using an opticaladhesive. The front reflector 210 and support layer can be attached tothe backlight using any suitable technique, e.g., those techniquesdescribed in U.S. Patent Application No. 60/947,776 (Thunhorst et al.),entitled OPTICALLY TRANSMISSIVE COMPOSITE FILM FRAME.

In some embodiments, the front reflector 210 can be attached to the LCpanel. For example, the front reflector can be attached to lowerabsorbing polarizer 158 of FIG. 1, which is in turn attached to panelplate 154.

As stated herein, the front reflector 210 can include any suitablefilm(s) and/or layer(s) that provide a partially reflective andpartially transmissive front reflector. In some embodiments, the frontreflector 210 can include one or more fiber polarizing films asdescribed, e.g., in U.S. Patent Publication No. 2006/0193577 (Ouderkirket al.), entitled REFLECTIVE POLARIZERS CONTAINING POLYMER FIBERS; U.S.patent application Ser. No. 11/468,746 (Ouderkirk et al.), entitledMULTILAYER POLARIZING FIBERS AND POLARIZERS USING SAME; and U.S. patentapplication Ser. No. 11/468,740 (Bluem et al.), entitled POLYMER FIBERPOLARIZERS. Other exemplary films that can be used for the frontreflector 210 include cholesteric polarizing films, birefringentpile-of-plates films, and birefringent polymer blends (e.g., DRPF,available from 3M Company).

The asymmetric reflective films of the present disclosure can bemanufactured using any suitable technique. See, e.g., U.S. Pat. No.6,783,349 (Neavin et al.), entitled APPARATUS FOR MAKING MULTILAYEROPTICAL FILMS. For example, FIGS. 9A-B illustrate one embodiment of amethod for making asymmetric reflective films of the present disclosure.Materials 900 and 902, selected to have suitably different opticalproperties, are heated above their melting and/or glass transitiontemperatures and fed into a multilayer feedblock 904. Typically, meltingand initial feeding is accomplished using an extruder for each material.For example, material 900 can be fed into an extruder 901 while material902 can be fed into an extruder 903. Exiting from the feedblock 904 is amultilayer flow stream 905. A layer multiplier 906 splits the multilayerflow stream, and then redirects and “stacks” one stream atop the secondto multiply the number of layers extruded. An asymmetric multiplier,when used with extrusion equipment that introduces layer thicknessdeviations throughout the stack, may broaden the distribution of layerthicknesses so as to enable the multilayer film to have layer pairscorresponding to a desired portion of the visible spectrum of light, andprovide a desired layer thickness gradient. If desired, skin layers 911may be introduced into the film by feeding resin 908 (for skin layers)to a skin layer feedblock 910.

The multilayer feedblock feeds a film extrusion die 912. Suitablefeedblocks are described, for example, in U.S. Pat. No. 3,773,882(Schrenk) and U.S. Pat. No. 3,884,606 (Schrenk). As an example, theextrusion temperature may be approximately 295 C.°, and the feed rateapproximately 10-150 kg/hour for each material. In some embodiments, itmay be preferred to have the skin layers 911 flowing on the upper andlower surfaces of the film as it goes through the feedblock and die.These layers serve to dissipate the large stress gradient found near thewall, leading to smoother extrusion of the optical layers. Typicalextrusion rates for each skin layer would be 2-50 kg/hr (1-40% of thetotal throughput). The skin material can be the same material as one ofthe optical layers or be a different material. An extrudate leaving thedie is typically in a melt form.

The extrudate is cooled on a casting wheel 916, which rotates pastpinning wire 914. The pinning wire pins the extrudate to the castingwheel. To achieve a clear film over a broad range of angles, one canmake the film thicker by running the casting wheel at a slow speed,which moves the reflecting band towards longer wavelengths. The film isoriented by stretching at ratios determined by the desired optical andmechanical properties. Longitudinal stretching can be done by pull rolls918. Transverse stretching can be done in a tenter oven 920. If desired,the film can be bi-axially oriented simultaneously. Stretch ratios ofapproximately 3-4 to 1 may be preferred, although ratios as small as 1to 1 and as large as 6 to 1 may also be appropriate for a given film.Stretch temperatures will depend on the type of birefringent polymerused, but 2° to 33° C. (5° to 60° F.) above its glass transitiontemperature would generally be an appropriate range. The film istypically heat set in the last two zones 922 of the tenter oven toimpart the maximum crystallinity in the film and reduce its shrinkage.Employing a heat set temperature as high as possible without causingfilm breakage in the tenter reduces the shrinkage during a heatedembossing step. A reduction in the width of the tenter rails by about1-4% also serves to reduce film shrinkage. If the film is not heat set,heat shrink properties are maximized, which may be desirable in somesecurity packaging applications. The film can be collected on winduproll 924.

In some applications, it may be desirable to use more than two differentpolymers in the optical layers of the multilayer film. In such a case,additional resin streams can be fed using similar means to resin streams900 and 902. A feedblock appropriate for distributing more than twolayer types analogous to the feedblock 904 could be used.

FIG. 9B shows a schematic perspective view of one embodiment offeedblock 904, which is enclosed in a housing 928. Within the housing928 resides a gradient plate 930. Residing in gradient plate 930 are atleast two flow channels, a first flow channel 932 and a second flowchannel 934. The flow channels are bounded by a combination of thegradient plate 930 and a feeder tube plate 940.

In the gradient plate 930, each flow channel is machined so that itscross-section has a central axis of symmetry, such as, e.g., a circle,square, or equilateral triangle. For ease of machining purposes, thesquare cross-section flow channel is preferably used. Along each flowchannel, the cross-sectional area can remain constant or can change. Thechange may be an increase or decrease in area, and a decreasingcross-section is typically referred to as a “taper.” A change incross-sectional area of the flow channels can be designed to provide anappropriate pressure gradient, which affects the layer thicknessdistribution of a multilayer optical film. Thus, the gradient plate canbe changed for different types of multilayer film constructions.

When the cross-sectional area of the flow channels is made to remainconstant, a plot of layer thickness vs. layer number is non-linear anddecreasing. For a given polymer flow, there exists at least onecross-sectional tapering profile which will result in a linear,decreasing dependency of layer thickness upon layer number, which issometimes preferred. The taper profile can be found by one reasonablyskilled in the art using reliable rheological data for the polymer inquestion and polymer flow simulation software known in the art, andshould be calculated on a case by case basis.

Referring again to FIG. 9B, the feedblock 904 further contains a feedertube plate 940 that has a first set of conduits 942 and a second set ofconduits 944, each set in fluid communication with flow channels 932 and934 respectively. As used herein, “conduits” are also referred to as“side channel tubes.” Residing proximate conduits 942 and 944 are axialrod heaters 946, used to provide heat to the resin flowing in theconduits. If desired, temperature can be varied in zones along thelength of the axial rod heaters. Each conduit feeds its own respectiveslot die 956, which has an expansion section and a slot section. Theexpansion section typically resides in the feeder tube plate 940. Ifdesired, the slot section can reside in a slot plate 950. As usedherein, the term “slot die” is synonymous with “layer slot.” The firstset of conduits 942 is interleaved with the second set of conduits 944to form alternating layers.

In use, for example, resin A and resin B would be delivered directly tothe flow channels 932 and 934. As the melt stream A and melt stream Btravel down the flow channels in the gradient plate 930, each meltstream is bled off by the conduits. Because the conduits 942 and 944 areinterleaved, they begin the formation of alternating layers, such as,for example, ABABAB. Each conduit has its own slot die to begin theformation of an actual layer. The melt stream exiting the slot diecontains a plurality of alternating layers. The melt stream is fed intoa compression section (not shown) where the layers are compressed andalso uniformly spread out transversely. Special thick layers known asprotective boundary layers (PBLs) may be fed nearest to the feedblockwalls from any of the melt streams used for the optical multilayerstack. The PBLs can also be fed by a separate feed stream after thefeedblock. The PBLs function to protect the thinner optical layers fromthe effects of wall stress and possible resulting flow instabilities.

In some embodiments, the asymmetric reflective films of the presentdisclosure can be manufactured without the use of a multiplier (e.g.,multiplier 906). Although multipliers greatly simplify the generation ofa large number of optical layers, they may impart distortions to eachresultant packet of layers that are not identical for each packet. Forthis reason, any adjustment in the layer thickness profile of the layersgenerated in the feedblock is not the same for each packet, i.e., allpackets cannot be simultaneously optimized to produce a uniform smoothspectrum free of spectral disruptions. Thus, an optimum profile and lowtransmission color reflector can be difficult to make using multi-packetfilms manufactured using multipliers. If the number of layers in asingle packet generated directly in a feedblock do not providesufficient reflectivity, then two or more such films can be attached toincrease the reflectivity.

When the multiplier is removed from the method of FIG. 9A, the axial rodheaters 946 can be used to control the layer thickness values ofcoextruded polymer layers as is further described, e.g., in U.S. Pat.No. 6,783,349. Such axial rod heaters can be utilized both formaintaining constant temperature in the feedblock and for creating atemperature gradient of up to about 40° C. In some embodiments, theaxial rod heaters are placed in a bore through the feedblock andoriented in a direction normal to the layer plane, preferably very nearan imaginary line through the points where each side channel tube feedsa slot die. More preferably, in the case of coextrusion of a firstpolymer and a second polymer, the bores for the axial rod heaters willbe located both near an imaginary line through the points where eachside channel tube feeds a slot die, and also equidistant from the sidechannel tubes carrying the first polymer and the side channel tubescarrying the second polymer. Further, the axial rod heaters arepreferably of a type that can provide a temperature gradient or amultiplicity of discrete temperatures along its length, either byvariation in electrical resistance along its length, or by multi-zonecontrol, or by other means known in the art. Such axial rod heaters cancontrol layer thickness and gradient layer thickness distribution, whichis especially important in controlling the positions and profiles ofreflection bands as described, e.g., in U.S. Pat. No. 6,157,490(Wheatley et al.), entitled OPTICAL FILM WITH SHARPENED BANDEDGE; andU.S. Pat. No. 6,531,230 (Weber et al.), entitled COLOR SHIFTING FILM.

The feedblock 904 is configured such that all layers in the film stackare directly controlled by an axial rod heater 946. Layer thicknessprofile can be monitored during the process by using any suitablethickness measuring technique, e.g., atomic force microscopy,transmission electron microscopy, or scanning electron microscopy. Thelayer thickness profile can also be modeled optically using any suitabletechnique, and then the axial rod heaters can be adjusted based on thedifference between the measured layer profile and the desired layerprofile.

Although not as accurate in general as an AFM, the layer profile canalso be quickly estimated by integrating the optical spectrum(integrating the—Log(1-R) vs. wavelength spectrum). This follows fromthe general principle that the spectral shape of a reflector can beobtained from the derivative of the layer thickness profile, providedthe layer thickness profile is monotonically increasing or decreasingwith respect to layer number.

The basic process for layer thickness profile control involvesadjustment of axial rod zone power settings based on the difference ofthe target layer thickness profile and the measured layer profile. Theaxial rod power increase needed to adjust the layer thickness values ina given feedblock zone can first be calibrated in terms of watts of heatinput per nanometer of resulting thickness change of the layersgenerated in that heater zone. Fine control of the spectrum is possibleusing 24 axial rod zones for 275 layers. Once calibrated, the necessarypower adjustments can be calculated once given a target profile and ameasured profile. The procedure is repeated until the two profilesconverge.

For example, a film of 275 layers was made according to the abovetechnique, using a coPEN with indices of nx1=1.82, ny1=1.61, nz=1.50,and a mixture of PCTG with polycarbonate with index n2=1.57 for alldirections. The measured transmission spectrum for p-polarized light at60° angle of incidence on the pass axis is shown in FIG. 10 as curve1002. Also shown is the transmission of the block axis as curve 1004.Note that both the block and the pass axis spectra have relativelyconstant transmission over a very broad band.

Returning to FIG. 2, the backlight 200 also includes back reflector 220that, along with the front reflector 210, form the hollow lightrecycling cavity 202. The back reflector 220 is preferably highlyreflective. For example, the back reflector 220 can have an on-axisaverage reflectivity for visible light emitted by the light sources ofat least 90%, 95%, 98%, 99%, or more for visible light of anypolarization. Such reflectivity values also can reduce the amount ofloss in a highly recycling cavity. Such reflectivity values encompassall visible light reflected into a hemisphere, i.e., such values includeboth specular and diffuse reflections.

The back reflector 220 can be a predominantly specular, diffuse, orcombination specular/diffuse reflector, whether spatially uniform orpatterned. In some embodiments, the back reflector 220 can be asemi-specular reflector as is further described herein. See also PCTPatent Application No. PCT/US2008/64115, entitled RECYCLING BACKLIGHTSWITH BENEFICIAL DESIGN CHARACTERISTICS; and U.S. patent application Ser.No. 11/467,326 (Ma et al.), entitled BACKLIGHT SUITABLE FOR DISPLAYDEVICES. In some cases, the back reflector 220 can be made from a stiffmetal substrate with a high reflectivity coating, or a high reflectivityfilm laminated to a supporting substrate. Suitable high reflectivitymaterials include Vikuiti™ Enhanced Specular Reflector (ESR) multilayerpolymeric film available from 3M Company; a film made by laminating abarium sulfate-loaded polyethylene terephthalate film (2 mils thick) toVikuiti™ ESR film using a 0.4 mil thick isooctylacrylate acrylic acidpressure sensitive adhesive, the resulting laminate film referred toherein as “EDR II” film; E-60 series Lumirror™ polyester film availablefrom Toray Industries, Inc.; porous polytetrafluoroethylene (PTFE)films, such as those available from W. L. Gore & Associates, Inc.;Spectralon™ reflectance material available from Labsphere, Inc.; Miro™anodized aluminum films (including Miro™ 2 film) available from AlanodAluminum-Veredlung GmbH & Co.; MCPET high reflectivity foamed sheetingfrom Furukawa Electric Co., Ltd.; White Refstar™ films and MT filmsavailable from Mitsui Chemicals, Inc.; and 2×TIPS (see Examples fordescription).

The back reflector 220 can be substantially flat and smooth, or it mayhave a structured surface associated with it to enhance light scatteringor mixing. Such a structured surface can be imparted (a) on the surfaceof the back reflector 220, or (b) on a transparent coating applied tothe surface. In the former case, a highly reflecting film may belaminated to a substrate in which a structured surface was previouslyformed, or a highly reflecting film may be laminated to a flat substrate(such as a thin metal sheet, as with Vikuiti™ Durable Enhanced SpecularReflector-Metal (DESR-M) reflector available from 3M Company) followedby forming the structured surface, such as with a stamping operation. Inthe latter case, a transparent film having a structured surface can belaminated to a flat reflective surface, or a transparent film can beapplied to the reflector and then afterwards a structured surface can beimparted to the top of the transparent film.

For those embodiments that include a direct-lit configuration (e.g.,backlight 1600 of FIG. 16), the back reflector can be a continuousunitary (and unbroken) layer on which the light source(s) are mounted,or it can be constructed discontinuously in separate pieces, ordiscontinuously insofar as it includes isolated apertures, through whichlight sources can protrude, in an otherwise continuous layer. Forexample, strips of reflective material can be applied to a substrate onwhich rows of light sources are mounted, each strip having a widthsufficient to extend from one row of light sources to another and havinga length dimension sufficient to span between opposed borders of thebacklight's output area.

The backlight 200 can also include one or more side reflectors 250located along at least a portion of the outer boundary of the backlight200 that are preferably lined or otherwise provided with highreflectivity vertical walls to reduce light loss and improve recyclingefficiency. The same reflective material used for the back reflector 220can be used to form these reflectors, or a different reflective materialcan be used. In some embodiments, the side reflectors 250 and backreflector 220 can be formed from a single sheet of material.

The side reflectors 250 can be vertical, or alternatively, one or morereflectors can be tilted. Also, the reflective material for the sidereflectors 250 can be the same or different as the material used for theback reflector 220. Specular, semispecular, diffuse materials can beused on for the side reflectors 250. Refractive structures can be usedon or adjacent to side reflectors 250 to achieve a desired reflectionprofile. Wall material and inclination can be chosen to adjust thebrightness profile.

The light sources 230 are shown schematically. In most cases, thesesources 230 are compact light emitting diodes (LEDs). In this regard,“LED” refers to a diode that emits light, whether visible, ultraviolet,or infrared. It includes incoherent encased or encapsulatedsemiconductor devices marketed as “LEDs”, whether of the conventional orsuper radiant variety. If the LED emits non-visible light such asultraviolet light, and in some cases where it emits visible light, it ispackaged to include a phosphor (or it may illuminate a remotely disposedphosphor) to convert short wavelength light to longer wavelength visiblelight, in some cases yielding a device that emits white light. An “LEDdie” is an LED in its most basic form, i.e., in the form of anindividual component or chip made by semiconductor processingprocedures. The component or chip can include electrical contactssuitable for application of power to energize the device. The individuallayers and other functional elements of the component or chip aretypically formed on the wafer scale, and the finished wafer can then bediced into individual piece parts to yield a multiplicity of LED dies.More discussion of packaged LEDs, including forward-emitting andside-emitting LEDs, is provided herein.

Multicolored light sources, whether or not used to create white light,can take many forms in a backlight, with different effects on color andbrightness uniformity of the backlight output area. In one approach,multiple LED dies (e.g., a red, a green, and a blue light emitting die)are all mounted in close proximity to each other on a lead frame orother substrate, and then encased together in a single encapsulantmaterial to form a single package, which may also include a single lenscomponent. Such a source can be controlled to emit any one of theindividual colors, or all colors simultaneously. In another approach,individually packaged LEDs, with only one LED die and one emitted colorper package, can be clustered together for a given recycling cavity, thecluster containing a combination of packaged LEDs emitting differentcolors such as blue/yellow or red/green/blue. In still another approach,such individually packaged multicolored LEDs can be positioned in one ormore lines, arrays, or other patterns.

LED efficiency is temperature dependent and generally decreases withincreasing temperature. This efficiency decrease may be different fordifferent types of LEDs. For example, red LEDs exhibit a significantlygreater efficiency decrease than blue or green. Various embodiments ofthe present disclosure can be used to mitigate this effect if the morethermally sensitive LEDs are thermally isolated so that they have alower watt density on the heat sink, and/or are not subject to heattransfer from the other LEDs. In a conventional backlight, locating acluster of one color of LEDs would result in poor color uniformity. Inthe present disclosure, the color of, for example a cluster of reds, canmix well with green and blue LEDs to form white. A light sensor andfeedback system can be used to detect and control the brightness and/orcolor of light from the LEDs. For example, a sensor can be located nearindividual or clusters of LEDs to monitor output and provide feedback tocontrol, maintain, or adjust a white point or color temperature. It maybe beneficial to locate one or more sensors along the edge or within thehollow cavity to sample the mixed light. In some instances it may bebeneficial to provide a sensor to detect ambient light outside thedisplay in the viewing environment, for example, the room in which thedisplay is located. In such a case, control logic can be used toappropriately adjust the display light source output based on ambientviewing conditions. Many types of sensors can be used such aslight-to-frequency or light-to-voltage sensors available from TexasAdvanced Optoelectronic Solutions, Plano, Tex. Additionally, thermalsensors can be used to monitor and control the output of LEDs. All ofthese techniques can be used to adjust base on operating conditions andbased on compensation of component aging over time. Sensors can be usedfor dynamic contrast or field sequential systems to supply feedbacksignals to the control systems.

If desired, other visible light emitters such as linear cold cathodefluorescent lamps (CCFLs) or hot cathode fluorescent lamps (HCFLs) canbe used instead of or in addition to discrete LED sources asillumination sources for the disclosed backlights. In addition, hybridsystems such as, for example, (CCFL/LED), including cool white and warmwhite, CCFL/HCFL, such as those that emit different spectra, may beused. The combinations of light emitters may vary widely, and includeLEDs and CCFLs, and pluralities such as, for example, multiple CCFLs,multiple CCFLs of different colors, and LEDs and CCFLs.

For example, in some applications it may be desirable to replace the rowof discrete light sources with a different light source such as a longcylindrical CCFL, or with a linear surface emitting light guide emittinglight along its length and coupled to a remote active component (such asan LED die or halogen bulb), and to do likewise with other rows ofsources. Examples of such linear surface emitting light guides aredisclosed in U.S. Pat. No. 5,845,038 (Lundin et al.) and U.S. Pat. No.6,367,941 (Lea et al.). Fiber-coupled laser diode and othersemiconductor emitters are also known, and in those cases the output endof the fiber optic waveguide can be considered to be a light source withrespect to its placement in the disclosed recycling cavities orotherwise behind the output area of the backlight. The same is also trueof other passive optical components having small emitting areas such aslenses, deflectors, narrow light guides, and the like that give offlight received from an active component such as a bulb or LED die. Oneexample of such a passive component is a molded encapsulant or lens of aside-emitting packaged LED.

Any suitable side-emitting LED can be used for one or more lightsources, e.g., Luxeon™ LEDs (available from Lumileds, San Jose, Calif.),or the LEDs described, e.g., in U.S. patent application Ser. No.11/381,324 (Leatherdale et al.), entitled LED PACKAGE WITH CONVERGINGOPTICAL ELEMENT; and U.S. patent application Ser. No. 11/381,293 (Lu etal.), entitled LED PACKAGE WITH WEDGE-SHAPED OPTICAL ELEMENT. Otheremission patterns may be desired for various embodiments describedherein. See, e.g., U.S. Patent Publication No. 2007/0257270 (Lu et al.),entitled LED PACKAGE WITH WEDGE-SHAPED OPTICAL ELEMENT.

In some embodiments where the backlights are used in combination with adisplay panel (e.g., panel 150 of FIG. 1), the backlight 200continuously emits white light, and the LC panel is combined with acolor filter matrix to form groups of multicolored pixels (such asyellow/blue (YB) pixels, red/green/blue (RGB) pixels,red/green/blue/white (RGBW) pixels, red/yellow/green/blue (RYGB) pixels,red/yellow/green/cyan/blue (RYGCB) pixels, or the like) so that thedisplayed image is polychromatic. Alternatively, polychromatic imagescan be displayed using color sequential techniques, where, instead ofcontinuously back-illuminating the LC panel with white light andmodulating groups of multicolored pixels in the LC panel to producecolor, separate differently colored light sources within the backlight200 (selected, for example, from red, orange, amber, yellow, green,cyan, blue (including royal blue), and white in combinations such asthose mentioned above) are modulated such that the backlight flashes aspatially uniform colored light output (such as, for example, red, thengreen, then blue) in rapid repeating succession. This color-modulatedbacklight is then combined with a display module that has only one pixelarray (without any color filter matrix), the pixel array being modulatedsynchronously with the backlight to produce the whole gamut ofachievable colors (given the light sources used in the backlight) overthe entire pixel array, provided the modulation is fast enough to yieldtemporal color-mixing in the visual system of the observer. Examples ofcolor sequential displays, also known as field sequential displays, aredescribed in U.S. Pat. No. 5,337,068 (Stewart et al.) and U.S. Pat. No.6,762,743 (Yoshihara et al.). In some cases, it may be desirable toprovide only a monochrome display. In those cases the backlights 200 caninclude filters or specific sources that emit predominantly in onevisible wavelength or color.

In some embodiments, the light sources can include one or more polarizedsources. In such embodiments, it may be preferred that a polarizationaxis of the polarized sources is oriented such that it is substantiallyparallel with a pass axis of the front reflector; alternatively it maybe preferred that the source polarization axis is substantialperpendicular to the pass axis of the front reflector. In otherembodiments, the polarization axis may form any suitable angle relativeto the pass axis of the front reflector.

In some embodiments, e.g., direct-lit backlights such as the embodimentillustrated in FIG. 16, the light sources may be positioned on the backreflector; alternatively, the light sources may be spaced apart from theback reflector. In other embodiments, the light sources may includelight sources that are positioned on or attached to the back reflector,e.g., as described in co-owned and copending U.S. patent applicationSer. Nos. 11/018,608; 11/018,605; 11/018,961; and 10/858,539.

The light sources 230 may be positioned in any suitable arrangement.Further, the light sources 230 can include light sources that emitdifferent wavelengths or colors of light. For example, the light sourcesmay include a first light source that emits a first wavelength ofillumination light, and a second light source that emits a secondwavelength of illumination light. The first wavelength may be the sameas or different from the second wavelength. The light sources 230 mayalso include a third light source that emits a third wavelength oflight. See, e.g., PCT Patent Application No. PCT/US2008/64129, entitledWHITE LIGHT BACKLIGHTS AND THE LIKE WITH EFFICIENT UTILIZATION OFCOLORED LED SOURCES. In some embodiments, the various light sources 230may produce light that, when mixed, provides white illumination light toa display panel or other device. In other embodiments, the light sources230 may each produce white light.

Further, in some embodiments, light sources that at least partiallycollimate the emitted light may be preferred. Such light sources caninclude lenses, extractors, shaped encapsulants, or combinations thereofof optical elements to provide a desired output into the hollow lightrecycling cavity of the disclosed backlights. Further, the backlights ofthe present disclosure can include injection optics that partiallycollimate or confine light initially injected into the recycling cavityto propagation directions close to a transverse plane (the transverseplane being parallel to the output area of the backlight), e.g., aninjection beam having a full angle-width (about the transverse plane) athalf maximum power (FWHM) in a range from 0 to 90 degrees, or 0 to 60degrees, or 0 to 30 degrees, 0 to 15 degrees, or 0 to 10 degrees orless. Suitable injector shapes include wedge, parabolic, compoundparabolic, etc.

In general, the FWHM value for the light emitted into the cavity 202 bythe one or more light sources 230 can be controlled to provide thedesired collimation. Any suitable value of FWHM can be provided usingany suitable technique. Further, the direction of the injected light canalso be controlled to provide desired transport characteristics. Forexample, light from the one or more light sources can be directed intothe cavity at any suitable angle to the transverse plane. In someembodiments, the injected light can be directed in a direction towardthe back reflector 220.

In some embodiments of the present disclosure it may be preferred thatsome degree of diffusion be provided within the hollow light recyclingcavity. Such diffusion can provide more angular mixing of light withinthe cavity, thereby helping to spread the light within the cavity andprovide greater uniformity in the light directed out of the cavitythrough the output surface. In other words, the recycling optical cavitycontains a component that provides the cavity with a balance of specularand diffuse characteristics, the component having sufficient specularityto support significant lateral light transport or mixing within thecavity, but also having sufficient diffusivity to substantiallyhomogenize the angular distribution of steady state light propagationwithin the cavity, even when injecting light into the cavity only over anarrow range of propagation angles. Additionally, recycling within thecavity must result in a degree of randomization of reflected lightpolarization relative to the incident light polarization state. Thisallows for a mechanism by which unusable polarization light can beconverted by recycling into usable polarization light. The diffusion canbe provided by one or both of the front and back reflectors, the sidereflectors, or by one or more layers positioned between the front andback reflectors as is further described herein.

In some embodiments, the diffusion provided within the cavity caninclude semi-specular diffusion. As used herein, the term “semi-specularreflector” refers to a reflector that reflects substantially moreforward scattering than reverse scattering. Similarly, the term“semi-specular diffuser” refers to a diffuser that does not reverse thenormal component of the incident ray for a substantial majority of theincident light, i.e., the light is substantially transmitted in theforward (z) direction and scattered to some degree in the x and ydirections. In other words, semi-specular reflectors and diffusersdirect the light in a substantially forward direction and thus are verydifferent from Lambertian components that redirect light rays equally inall directions. Semi-specular reflectors and diffusers can exhibitrelatively wide scattering angles; alternatively, such reflectors anddiffusers can exhibit only small amounts of light deflection outside thespecular direction. See, e.g., PCT Patent Application No.PCT/US2008/64115, entitled RECYCLING BACKLIGHTS WITH SEMI-SPECULARCOMPONENTS.

Semi-specular reflectors can assist the lateral spreading of lightacross the cavity while still providing adequate mixing of polarizationand light ray directions. For example, FIG. 11 is a schematiccross-section view of a portion of a backlight 1100 that includes adiffusely reflective front reflector 1120 and a diffusely reflectiveback reflector 1130. Both the front and back reflectors are Lambertianreflectors, i.e., both reflectors reflect light substantially equally inall directions. The front reflector 1120 is also partially transmissive.As such, the front and back reflectors 1120, 1130 direct equal amountsof light in the forward and reverse direction on each reflection, whichgreatly diminishes the forward directed component of a light ray afterseveral reflections.

As seen in FIG. 11, ray 1160 is incident on the front reflector 1120 andis diffusely reflected. At least a portion of the incident light 1160 istransmitted through the front reflector 1120. Light 1162, which is aportion of the diffusely reflected light, is subsequently incident onthe back reflector 1130, where it is diffusely reflected. Thecombination of diffusely reflective front and back reflectors 1120, 1130may prevent a sufficient spreading of light within the backlight 1100because a substantial amount of light 1160 is directed backwards in adirection opposite that of the propagation direction of the light 1160.Because front reflector 1120 is partially transmissive, a potentiallydisproportionate amount of light may be extracted by the front reflectoron one end of the backlight 1100.

In contrast to the embodiment illustrated in FIG. 11, FIG. 12 is aschematic cross-section view of a portion of another embodiment of abacklight 1200 that includes a specularly reflective front reflector1220 and a semi-specular back reflector 1230. Ray 1260 is incident onthe reflector 1220 where at least a portion of light 1262 is specularlyreflected toward the back reflector 1230 and a portion 1264 of light istransmitted. In turn, light 1262 is semi-specularly reflected by theback reflector 1230 such that a substantial portion of light continuesto propagate in the forward direction.

Any suitable semi-specular material or materials can be used for thefront and back reflectors of the present disclosure. See, e.g., PCTPatent Application No. PCT/US2008/64115, entitled RECYCLING BACKLIGHTSWITH SEMI-SPECULAR COMPONENTS.

Further, for example, the semi-specular back reflectors can include apartially transmitting specular reflector on a high reflectance diffusereflector. Suitable partially transmitting specular reflectors includeany of the partially transmitting reflective films described herein,e.g., symmetric or asymmetric reflective films. Suitable highreflectance diffuse reflectors include EDR II film (available from 3M);porous polytetrafluoroethylene (PTFE) films, such as those availablefrom W. L. Gore & Associates, Inc.; Spectralon™ reflectance materialavailable from Labsphere, Inc.; MCPET high reflectivity foamed sheetingfrom Furukawa Electric Co., Ltd.; and White Refstar™ film available fromMitsui Chemicals, Inc.

In another embodiment, a semi-specular back reflector can include apartial Lambertian diffuser on a high reflectance specular reflector.Alternatively, a forward scattering diffuser on a high reflectancespecular reflector can provide a semi-specular back reflector.

The front reflector can be made semi-specular with constructions thatare similar to the back reflector. For example, a partial reflectingLambertian diffuser can be combined with a partial specular reflector.Alternatively, a forward scattering diffuser can be combined with apartial specular reflector. Further, the front reflector can include aforward scattering partial reflector. In other embodiments, any of theabove-described front reflectors can be combined to provide asemi-specular front reflector. For example, the front reflector caninclude an asymmetric reflective film having refractive structurespositioned on or in a major surface of the reflector that faces thecavity, e.g., front reflector 810 or 820 of FIGS. 8B and 8C.

In some embodiments, additional optical components can be inserted intothe recycling cavity between the front and back reflectors, and suchadditional components may be tailored to provide the desired degree ofsemi-specularity to the cavity. For example, a semi-specular diffusingfilm can be suspended in the cavity between the front and backreflectors, both of which can be specular or semi-specular. Although itis often desirable to minimize the number of components in the cavity,the use of a third component can sometimes provide a higher efficiencycavity by allowing for the minimal loss design of the front or backreflector.

The mixing of light rays in the cavity with forward scattering elementscan be accomplished in several ways. It can be done by diffusingelements that are either an integral part of the front or back reflectoror are laminated to the front or back reflector, or by use of a separatediffusing sheet placed anywhere between the two. Combinations of any ofthese options are also possible. The choices depend on the relativeimportance of matters such as optical losses, component cost, andconvenience of manufacturing. The diffusing element may be attached toor an integral part of either the front or back reflector, or an air gapmay be provided between the diffuser and the reflectors.

Whether the diffuser is an integral part of either reflector, orlaminated to either reflector, or placed in the cavity as a separatecomponent, the overall desired optical performance is one with anangular spreading function that is substantially narrower than aLambertian distribution for a ray that completes one round trip passagefrom the back reflector to the front and back again. A semi-specularreflector can have characteristics of both a specular and a Lambertianreflector or can be a well defined Gaussian cone about the speculardirection. The performance depends greatly on how it is constructed.See, e.g., PCT Patent Application No. PCT/US2008/64115.

As mentioned herein, backlights of the present disclosure are hollow,i.e., the lateral transport of light within the cavity occurspredominantly in air, vacuum, or the like rather than in an opticallydense medium such as acrylic or glass. Historically, solid light guideshave generally been used for the thinnest backlights and, except forvery small displays such as handheld devices, have been illuminated withlinearly continuous light sources such as cold cathode fluorescentlights (CCFLs). Solid light guides can provide low loss transport oflight and specular reflections at the top and bottom surfaces of thelight guide via the phenomenon of total internal reflection (TIR) oflight. As described elsewhere in this application, the specularreflection of light provides the most efficient lateral transport oflight within a light guide. Extractors placed on the top or bottomsurface of a solid light guide redirect the light in order to direct itout of the light guide, creating in essence, a partial reflector.

However, solid light guides present several problems for large displays,such as cost, weight, and uniformity of light. The problem withuniformity for large area displays has increased with the advent ofseparate RGB colored LEDs, which are effectively point sources of lightcompared to the area of the output face of the backlight. The highintensity point sources cause uniformity problems with conventionaldirect-lit backlights as well as edge-lit systems that utilize solidlight guides. The uniformity problems can be greatly reduced if a hollowlight guide can be made that also provides for significant lateraltransport of light as in a solid light guide. In some cases forpolarization and light ray angle recycling systems, a hollow cavity canbe more proficient at spreading light laterally across a display facethan a solid cavity.

As previously described herein, the use of highly reflective front andback reflectors requires that the light losses in the recycling cavitiesby such reflectors are minimized. Losses in the cavity arise frommultiple sources such as absorption of light by the front partialreflector, the back reflector, the edge faces and corners, the lightsources and their support structure and/or their injection ports, aswell as losses in other components such as diffuser sheets or otherlight control films that may be placed inside the cavity or are a partof the partial reflector. Each reflection and transmission of everylight ray for each component of the cavity results in some loss of lightintensity. Therefore, overall losses for light propagating in therecycling cavity may be kept extraordinarily low, for example, both byproviding a substantially enclosed cavity of low absorptive loss,including low loss front and back reflectors as well as side reflectors,and by keeping losses associated with the light sources very low, forexample, by ensuring the cumulative emitting area of all the lightsources is a small fraction of the backlight output area

Losses that occur only once, e.g., in a component on the top side of thefront reflector, that the light traverses only once, lower the overallbrightness by a simple fraction equal to their percent absorption. Asshown herein, the repeated losses that occur for multiple reflections ofa light ray can create large losses of light intensity. Sides andcorners of the cavity can be sealed with mirror films if desired. Verythin multilayer polymeric reflectors that can be applied as a tape canbe used for that purpose. Where space is less of an issue, thickerdiffuse reflectors can be applied to seal the sides and corners of acavity.

A potential loss for polarized LCD backlights is light of the wrongpolarization that is transmitted by the front reflector, which is thenabsorbed by the lower polarizer on the LC panel. This loss can beminimized by utilizing a front reflector that transmits mainly polarizedlight, and by maximizing the reflectivity of the block axis of the frontreflector.

For any given level of partial reflectivity of the front reflector, theoutput of the system depends greatly on the loss value per transit ofthe cavity. This loss value is most easily characterized as an averageloss of all rays with all components in the cavity. For several reasons,the characteristic loss value is difficult to estimate by makingindividual loss measurements on all components. The losses typicallydepend on the angle of incidence of the light ray, and the relativenumber of rays that pass through or reflect off each component in acavity such as the side reflectors and light sources within the cavity.

The most direct way to estimate the characteristic loss value for theaggregate system is to measure the amount of light the cavity emitscompared to the light that all of its light sources emit. This ratio istypically called the cavity efficiency. Optical modeling of thesecavities can be helpful in understanding the relative importance of theindividual component and aggregate loss values of a cavity. The totalloss in the cavity depends on the reflectivity of the front reflectordue to the creation of multiple reflections and therefore multiplelosses for a given light ray. If the reflectivity of the front reflectoris increased, the average number of reflections of an average light rayincreases in the system.

A simple multi-bounce model for light reflecting between two reflectorsillustrates this function. For example, FIG. 13 is a graph of thefractional output of a cavity versus 1 minus the Cavity Loss for frontreflectors with various on-axis average reflectivities for lightpolarized parallel to the pass axis of the front reflector. The backreflector was assumed to have 100% reflection. A cavity loss value isassigned for each transit of a light ray from the front reflector to theback reflector. The model assumes a constant ray angle, i.e., it is aone dimensional model. The loss value represents potential loss valuesin both the front and the back reflector, as well as in any othercomponent in between. Curve 1302 represents a front reflector having anon-axis reflectivity of 85%, curve 1304 represents a front reflectorhaving an on-axis reflectivity of 75%, curve 1306 represents a frontreflector having an on-axis reflectivity of 50%, and curve 1308represents a front reflector having an on-axis reflectivity of 30%.

For increasing reflectance values of the partial reflector, the CavityLoss value becomes increasingly important in the efficiency of thecavity. For values above R=50% for the front reflector, the cavityoutput becomes substantially non-linear with respect to thecharacteristic cavity loss value, and the cavity loss value ispreferably less than 10%. (i.e., 1 minus the Cavity Loss should begreater than 0.90). In some embodiments, the back reflector can havegreater than 95% on-axis average reflectance as the total losses in thereflectors, light sources, edges, corners, etc. should be less than 10%.

A more complex cavity model for polarized backlights shows a similartrend. FIG. 14 illustrates the on-axis polarized output versus thereflectivity of the front reflector for various values. This modelcalculates the multi-bounce loss for all ray angles and polarizationstates within a cavity. Since the reflectivity of the front reflector ismodeled as a function of angle of incidence and polarization, theon-axis brightness of the cavity can be estimated. If the frontreflector has very high reflectivity for light polarized parallel to oneaxis (block axis R_(block)=99.9%), and the reflectivity of the otheraxis (R_(pass)) is varied by design, then the brightness of thebacklight, when viewed at normal incidence to its face, depends greatlyon the loss per pass in the cavity and the values of R_(pass). Theaverage loss per pass was varied from zero to 20% to obtain the variouscurves in FIG. 14 that show the on-axis brightness of the cavity foreach assumed value of cavity loss.

The on-axis brightness is a spatial average over the front face of thecavity, which in the limit of perfect uniformity is the brightness atany point over the front face. The front reflector was assumed to be anasymmetric reflective film, with indices similar to an oriented PEN/PMMAmultilayer stack. The high index PEN material includes the followingindices of refraction: nx1=1.82, ny1=variable, nz1=1.49. The indices forthe PMMA material are nx2=ny2=nz2=1.49. The value of R_(pass) was variedin the model by varying the y-index (pass axis direction) of the PENlayers while keeping all other indices constant in both layers of themultilayer repeat unit. Note that the output of the system is verysensitive to the loss function when the partial reflector has highvalues of reflectivity (R_(pass)). The model assumed an index differenceof 0.33 for the block axis of PEN/PMMA (i.e., Δx=0.33), a value that isreadily attainable with PEN or various coPENs in conjunction with PMMA.This assures a low loss due to block axis leakage. If the block axis hasa substantial leakage, it will contribute to the backlight loss functionsince light of this polarization will be absorbed by the bottomabsorbing polarizer of the LC panel.

Both the full and partially reflective films share the commonrequirement of low absorption losses. The absorptivity (A) of eitherfilm may be characterized by the expression A=1−T−R where T is thetransmissivity and R is the reflectivity as determined in a single passmeasurement. The absorption loss for the front reflector can beseparated into two components: the absorption loss upon reflection(A_(R)) and an additional absorption loss that can occur upontransmission (A_(T)) of a light ray. The latter occurs in many filmsthat have additional materials or material layers that do notparticipate in the actual reflection process, but which are necessary,e.g., for structural support or ease of manufacturing. It is importantthat these additional materials are not facing the inside of the cavitybut instead face outward whenever possible. In a recycling cavity, theabsorption loss upon reflection is more critical than the absorptionloss upon transmission. The latter only occurs once, whereas the formeroccurs with each bounce, or reflection of a light ray. As a result, a 5%absorption loss upon reflection can be multiplied to as much as a 50%loss or more, depending on the value of T for the front reflector. Bycontrast, an absorption loss of 5% upon transmission results in only a5% total loss. The two absorption loss values A_(R) and A_(T) for eachoptical component can be determined by measuring the reflectance fromeach side of the film, as well as the transmission of the film. Thisyields two equations and two unknowns: 1−T−R₁=A_(R) and 1−T−R₂=A_(T).

Losses can be minimized by using as few components as possible in thelight mixing cavity. For a low-loss hollow backlight that is designed topromote uniform lateral spreading of the light, attributes of thecomponents of the backlight cavity can include at least some of thefollowing:

-   -   1) high reflectivity (e.g., for the back reflector, edges,        corners, etc.; low absorption light sources and area surrounding        the light sources; adequate seal between top reflector and        backlight edges);    -   2) light ray mixing via semi-specular reflection or diffusion;    -   3) Partial reflectivity/transmissivity for the front reflector;    -   4) Efficient polarization output selection;    -   5) Efficient angle output selection;    -   6) Mechanical support (e.g., substrates and support posts) of        each component.

Separate components can be used to create one or more of these sixattributes, but it may be preferred to combine as many attributes intoas few components as possible to minimize the losses in the system. Forexample, multiple components can share a common substrate wheneverpossible. In addition, substrates can face outward from the cavitywhenever possible, so as to minimize multiple transmissions of lightthrough them.

Additional losses can also occur in the light sources or in theirsupporting substrates, or through the ports created to connect them totheir substrates and electrical connections. Ideally, only the emittingsurfaces of the light sources are exposed in the cavity, and all othersurfaces and components are covered with highly reflecting materials.The same is true for edge and corner losses. The remaining losses in thecavity occur mainly in the light reflecting and redirecting componentsthat provide the six attributes listed above.

As stated above, losses can be reduced with the use of multi-functionalcomponents. The term “multi-functional” refers to a component thatperforms the function of two or more separate components that are neededin the backlight. In this manner, components such as solid light guides,diffuser plates, reflective polarizers, prismatic films or otherbrightness enhancement films can be replaced by fewer components. Ingeneral, reducing the number of components used in a backlight canincrease the efficiency of the backlight and also reduce the depth ofthe backlight cavity. The thicknesses and types of materials selectedfor use in a component can also affect its loss values.

There are several types of materials and component designs that can beused to achieve one or more of the six attributes listed above. Forexample, the back reflector and/or the block axis of the front reflectorcan be made highly reflective. High reflectivity can be difficult toachieve with random reflective systems such as diffuse reflectors orpile-of-plates films. High reflectivity of only one polarization can bedifficult to achieve with such systems. Furthermore, high reflectivitywith a semi-specular scattering distribution can be difficult to achievewith diffuse reflectors. Constructive interference systems using ¼ wavethick layers can be made very reflective and much thinner, which isuseful in making thin backlights.

Examples in the industry of high reflectance films are diffuse filmslike micro-voided oriented polyester (PET) film from Toray Films, andspecular reflectors like ESR from 3M Co. The micro-voided film is about95% reflective (transmission is about 4%) and is about 0.2 mm thick. ESRis about 99% reflective and is only about 0.07 mm thick. Themicro-voided PET reflectivity is created by the index difference of airand oriented polyester, which is about 0.65. ESR is a multilayer film oforiented PEN and PMMA, with an index differential at normal incidence ofabout 0.26. Even though ESR has a much smaller index differential, itcan be made at least 99% reflective with a much thinner construction.

If a polarizing film with high block axis reflectance is desired, thenthe index differential between materials is limited by the birefringenceof the materials, since the indices need to be matched along one axis.For PEN, the birefringence is about 0.25, so diffuse reflectingpolarizers can be made with blended polymers having index differentialsof about 0.25. These constructions would therefore have to be muchthicker than the voided PET reflectors in order to be 95% reflective.The reflectivity may be limited by absorption losses in such thick filmssince highly birefringent materials often have relatively highabsorption coefficients compared to isotropic low index materials likeacrylic materials. The multilayer constructions have the additionalbenefit of being specular, which can enhance light transport across thehollow cavity. Cholesteric reflectors are also in the class of Braggreflectors and can be made very reflective and are inherentlypolarizing. A quarterwave retarder plate is needed to convert thecircularly polarized light to linear. To reduce losses to a minimum, theretarder can be placed on the outside face of the cavity so that thelight only passes through it once.

A silver metal back reflector can be 95% reflective if it is coated forcorrosion resistance. Higher reflectivities can be achieved with silverand other metals, most notably aluminum, if they are coated withdielectric thin films.

A wire grid polarizer can have a relatively high reflectivity for theblock axis if constructed properly. See, e.g., U.S. Pat. No. 6,122,103.There is some partial reflectivity for the pass axis, and this can beincreased with an additional multilayer reflector film stack.

The backlights of the present disclosure can include other types ofarrangements of light sources relative to the recycling cavity and theoutput surface. For example, FIG. 15 is a schematic cross-section viewof direct-lit backlight 1500. The backlight 1500 includes a frontreflector 1510 and a back reflector 1520 that form a hollow illuminationcavity 1502. The cavity 1502 includes an output surface 1504. Thebacklight 1500 also includes one or more light sources 1530 disposed toemit light into the cavity 1502. The backlight 1500 can optionallyinclude side reflectors 1550 surrounding at least a portion of theperiphery of the backlight 1500. All of the design considerations andpossibilities described herein regarding the front reflector 210, theback reflector 220, the one or more light sources 230, and the sidereflectors 250 of the backlight 200 of FIG. 2 apply equally to the frontreflector 1510, the back reflector 1520, the one or more light sources1530, and the side reflectors 1550 of the backlight 1500 illustrated inFIG. 15.

As previously stated herein, one or more films or layers can bepositioned between the front and back reflectors to further provideuniformity and/or efficiency. For example, FIG. 16 is a schematiccross-section view of one embodiment of a direct-lit backlight 1600 thatincludes a front reflector 1610, a back reflector 1620, and one or morelight sources 1620. The backlight 1600 also includes one or more sidereflectors 1650. All of the design considerations and possibilitiesregarding the front reflector 210, back reflector 220, light sources230, and side reflectors 250 of the embodiment illustrated in FIG. 2apply equally to the front reflector 1610, back reflector 1620, lightsources 1630, and side reflectors 1650 of the embodiment illustrated inFIG. 16. Although the side reflectors 1650 are depicted as extendingbetween the back reflector 1620 and the first diffuser 1660, the sidereflectors 1650 can also extend to any of the layers beyond the firstdiffuser 1660. In some embodiments, the side reflectors 1650 extend tothe front reflector 1610.

The backlight 1600 also includes a first diffuser 1660 and an optionalsecond diffuser 1670, both positioned between the front and backreflectors 1610, 1620. The first and second diffusers 1660, 1670 caninclude any suitable diffuser, e.g., diffuser plates, gain diffusers,bulk diffusers, etc. The first and second diffusers 1660, 1670 can bethe same or different diffusers.

The backlight 1600 also includes a brightness enhancing layer 1680positioned between the front and back reflectors 1610, 1620. Thebrightness enhancing layer 1680 can include any suitable brightnessenhancing layer or film, e.g., BEF (available from 3M Company).

In general, light from the light sources 1630 is diffused by the firstdiffuser 1660 and optional second diffuser 1670. For embodiments thatincluded a gain diffuser as the second diffuser 1670, diffuse light fromthe first diffuser 1660 is further diffused by the second diffuser 1670and collimated as well. The brightness enhancing layer 1680 further actsto collimate the diffuse light. Although depicted as including a singlebrightness enhancing layer 1680, the backlight 1600 can include a secondbrightness enhancing layer positioned between the first layer 1680 andthe front reflector 1610 to further collimate the light, e.g., in anorthogonal plane.

Light that is transmitted by the brightness enhancing layer 1680 ispartially reflected and partially transmitted by the front reflector1610. The reflected light is directed toward the back reflector 1620where it can be recycled within the cavity 1602. While some amount ofrecycling may occur between the front reflector 1610 and other films orlayers in the backlight 1600, a substantial portion of recycled light inthe cavity 1602 is recycled by the front and back reflectors 1610, 1620.

The backlights of the present disclosure can also include two or morezones, where each zone can provide different output characteristics suchas brightness, color, etc. Such zoned backlights can include verticalpartitions disposed between the front reflector and the back reflectorto partially or completely segment a hollow light recycling cavity intoseparate zones or sub-cavities. For example, FIG. 17 is a schematic planview of one embodiment of a zoned backlight 1700. The backlight 1700includes four zones 1702 a-d formed by vertical partitions that arearranged between the front and back reflectors as shown by the brokenlines to define distinct zones or cavities 1702 a-d. The partitions canbe made of (or covered with) a highly reflecting material such as theones listed as suitable for the back reflector; alternatively, one ormore partitions can also be partially transmissive. The partitions mayextend from the back reflector to the front reflector, or they mayextend only part of the way from the back reflector to the frontreflector or the front reflector to the back reflector, therebyproviding a small gap between one of the top edge and bottom edge of thepartition and the front reflector. The presence of such a gap,especially when used in conjunction with the front reflectors describedherein, can help hide the partition by reducing the local luminancenonuniformity that can be caused by the presence of the partition. Forexample, in some embodiments, the gap may be about 0.5 to 5 mm. In someembodiments, it may also be preferred that the top edge of the partition(the edge closest to the front reflector) be narrow so as to furtherreduce the visibility of the partition. Any suitable number of zones maybe provided in the backlight 1700.

The partitions can be separate elements that are appropriately placedwithin the backlight. Alternatively, the partitions can be formed in oneor both of the front and back reflectors. For example, the backreflector can be shaped or formed to provide partitions, e.g., as isdescribed in U.S. Patent Publication No. 2005/0265029 (Epstein et al.),entitled LED ARRAY SYSTEMS. Any suitable technique can be used to formpartitions in one or both of the front and back reflectors, e.g.,bending, thermoforming, stamping, pressure forming, etc. Light sourcesmay be disposed within each zone in a direct-lit configuration, alongthe periphery of each zone in an edge-lit configuration, or disposed toprovide a combination edge-lit/direct-lit backlight.

Further, the zones may be any suitable plan shape, e.g., rectangular,hexagonal, or other polygonal shapes; circles, ellipses, and any otherdesired shapes are contemplated. The geometry can be tailored to achievehigh efficiency and brightness and color uniformity in the backlight.

Backlights utilizing more than one of the disclosed recycling cavities,and particularly those having zones or arrays of distinct cavities, eachof which is illuminated by its own light source(s) which are separatelycontrolled or addressable relative to light source(s) in neighboringcavities, can be used with suitable drive electronics to support dynamiccontrast display techniques and color sequential display techniques, inwhich the brightness and/or color distribution across the output area ofthe backlight is intentionally non-uniform. Thus, different zones of theoutput area can be controlled to be brighter or darker than other zones,or the zones can emit in different colors, simply by appropriate controlof the different light sources in the different recycling cavities.

It may be desired to have light source redundancy within a zone. Forexample, one important concern of backlight designers is thatobjectionable non-uniformity could be seen by the consumer if anindividual light source fails. To mitigate this risk, one or more zonescan include two or more light sources so that if one source fails, therewould still be some minimum level of brightness within a zone. Thecontrol scheme could increase the brightness of the remaining lightsources within a zone to further compensate for the non-functioninglight source.

The various embodiments of backlights described herein can include alight sensor and feedback system to detect and control one or both ofthe brightness and color of light from the light sources. For example, asensor can be located near individual light sources or clusters ofsources to monitor output and provide feedback to control, maintain, oradjust a white point or color temperature. It may be beneficial tolocate one or more sensors along an edge or within the cavity to samplethe mixed light. In some instances it may be beneficial to provide asensor to detect ambient light outside the display in the viewingenvironment, for example, the room that the display is in. Control logiccan be used to appropriately adjust the output of the light sourcesbased on ambient viewing conditions. Any suitable sensor or sensors canbe used, e.g., light-to-frequency or light-to-voltage sensors (availablefrom Texas Advanced Optoelectronic Solutions, Plano, Tex.).Additionally, thermal sensors can be used to monitor and control theoutput of light sources. Any of these techniques can be used to adjustlight output based on operating conditions and compensation forcomponent aging over time. Further, sensors can be used for dynamiccontrast, vertical scanning or horizontal zones, or field sequentialsystems to supply feedback signals to the control system.

While not wishing to be bound by any particular theory, there are fourattributes for edge-lit backlights that can be selected with mutualcognizance to simultaneously achieve adequate efficiency, uniformity,and viewing angle. These are 1) the geometry of the cavity, 2) thereflective and transmissive properties of the emissive surface, 3) thereflective properties of the back reflector, and 4) the angulardistribution of light injected into the cavity at each illuminated edge.

The reflective and transmissive properties of the front reflector, andthe reflective properties of the back reflector, can be described by thebidirectional reflectivity and transmissivity distribution functions(BRDF and BTDF, respectively) of the former, and the BRDF of the latter.The BRDF describes the radiance reflected into every inward directionfor unit radiance incident in any outward direction. The BTDF describesanalogously the radiance transmitted into every outward direction forunit radiance incident in any outward direction. The totalreflectivity/transmissivity is the total power per unit areareflected/transmitted into all inward/outward directions for unit powerper unit area incident in any outward direction. The hemisphericalreflectivity/transmissivity is the total reflectivity/transmissivityaveraged over all directions of incidence. The hemisphericalreflectivity/transmissivity can be measured for wavelengths of incidentlight that are to be used with the backlight.

One or more of the embodiments of front reflectors described hereinexhibit a hemispherical reflectivity of at least 60%, with a totalreflectivity for directions of incidence substantially perpendicular tothe illuminated edge(s) and within approximately 30 degrees of grazingincidence of the front reflector that can be greater than thehemispherical reflectivity, and less than the hemispherical reflectivityfor directions of incidence substantially parallel to the illuminatededges or beyond 30 degrees of grazing.

For purposes of explanation, light within an edge-lit backlight can bethought of as falling into two classes of angular distributions. Thefirst angular distribution includes light within the recycling cavitythat is propagating in a direction that is substantially orthogonal tothe illuminated edge and at an incidence angle with the front reflectorof at least 60° measured from the front reflector normal. The secondangular distribution includes all propagating light within the cavitythat does not fall within the first angular distribution.

For example, FIG. 18 is a schematic view of the approximate dependenceof the total reflectivity upon the direction of incidence for one ormore embodiments of front reflectors described herein. The unit-radiuscircular domain depicts the projection of every outward direction ofincidence into the plane of the front reflector. For the embodimentillustrated in FIG. 18, the emission of the backlight will be bothsubstantially linearly polarized and most intense near normal and indirections deviating from normal parallel to the illuminated edge(s),and least intense in directions deviating from normal by more than 60degrees in directions perpendicular to the illuminated edge(s).

Light injected at the illuminated edge(s) is substantially whollyretained within the cavity if it strikes the emissive surfacesubstantially perpendicular to the illuminated edges and within 30degrees of grazing, i.e., light in the first angular distribution. It isotherwise partially retained within the cavity with the complementaryportion emitted. The radiance in each angular distribution diminishes asthe number of interactions with the front and back reflectors of thecavity increases, and, therefore, generally with increasing distancefrom the illuminated edge(s). The radiance in the second angulardistribution diminishes more rapidly than that in the first angulardistribution because of the relatively smaller values of the totalreflectivity of the front reflector. Since this second angulardistribution directly supplies the emission, the rapidity of its decayprimarily determines the overall uniformity of the backlight emission.

If the BRDFs of both the front and back reflectors are purely specular,then, assuming a cavity of uniform depth and specular edges, retainedlight will strike the front reflector with the same (or asymmetrically-equivalent) direction of incidence upon all subsequentencounters until it eventually interacts with the injection optics atthe illuminated edge(s). In this circumstance, there exists no leveragedmechanism by which light in either distribution can transfer to theother. Light in the first angular distribution remains substantiallytrapped within the cavity until it is eventually absorbed, therebydiminishing the efficiency of the backlight. Light in the second angulardistribution decays relatively rapidly because of the transmissionthrough the front reflector, thereby creating non-uniform emission.

If the BRDF of either the front reflector or back reflector possesses asignificant non-specular component, then retained light will strike thefront reflector with a potentially different direction of incidence uponeach subsequent encounter, thereby providing a mechanism for thetransfer of light from one distribution to the other. The average numberof bounces can be controlled, or equivalently the propagation distancenecessary to effect this transfer by controlling the degree ofnon-specularity in one or both BRDFs. In the presence of an appropriatedegree of conversion, light in the first angular distribution may begradually transferred to the latter as it propagates away from theilluminated edge(s), thereby avoiding the eventuality of its absorption,and at the same time slowing the decay of the second angulardistribution by providing a gradual influx distributed along the extentof the backlight perpendicular to its illuminated edge(s). The result isboth increased efficiency and improved uniformity.

The injection optics determine the angular distribution of light at theilluminated edge(s) of the cavity, and thus the initial population oflight within each of the two angular distributions. The initialpopulation, in turn, determines the sense and the magnitude of thetransfer of light between distributions in circumstances where one orboth BRDFs possess a non-specular component. Thus, for example, if lightis injected exclusively into the second angular distribution, the rateof decay of this distribution will increase beyond its relatively-highbaseline value due to a net transfer out of the second distribution intothe first with increasing distance from the illuminated edge(s). Whileincreasing the overall efficiency of emission, it will generally alsoresult in an overly-rapid decay, engendering a darkening at positionsremoved from the illuminated edge(s). If, at the opposite extreme, lightis injected exclusively into the first distribution, the net transferwill be out of the first into the second, and the second will decay lessrapidly than its baseline. Overall, the uniformity will improve,although at the expense of efficiency, and with the possibility ofdarkening near the illuminated edge(s) due to a local paucity of lightin the second distribution.

Imaging of the sources creates a second type of potentialnon-uniformity. Such imaging can create one or more bright bands orspots near the illuminated edge(s), which in the case of spots may alsoengender color non-uniformity when colored LEDs are used. In manycircumstances, the suppression of these non-uniformities trumps concernsover the gradual changes in intensity which occur across the separationbetween the illuminated edges.

Imaging can occur when 1) the angular distribution of injected lightcontains residual sharp features (engendered by spatially-discretesources), and 2) the injected light contributes directly to thebacklight emission. It can largely be eliminated by 1) eliminating thesharp features in the injected radiance, or 2) injecting lightexclusively into the first angular distribution. The former alternativeis usually accomplished by forcing multiple-bounces, possibly with somedegree of diffusion, within the injection optics. The latter alternativeis accomplished by partially-collimating designs, as described herein.

Uniformity is desired both perpendicular and parallel to the illuminatededge(s) of the display. Since the optical characteristics of the cavityare independent of position parallel to these edges, the injectionoptics can possess a cross section in any plane normal to theilluminated edge(s) that is independent of the position of the planealong these edge(s). That is, in some embodiments, the preferredinjection optics are translationally invariant along the illuminatededge(s). The collimation afforded by a translationally-invariant opticis completely specified by the angular substance of the emission of theoptic in the plane normal to the translational axis. If, for example,the in-plane emission is confined to within ψ degrees of any planeparallel to the emissive surface, the directions of incidence upon theemissive surface populated by the injected light will be all those forwhich |s_(perp)|≧√{square root over (1−s_(par) ²)} cos ψ.

It follows that the exclusive population of the first angulardistribution of light can be substantially accomplished by collimationof the injected light distribution injected by atranslationally-invariant optic. The degree of collimation required canbe dictated by the extent of the high-reflectivity domain of the frontreflector, and is evaluated once the BRDF of that surface is known. Forone or more embodiments of front reflectors described herein, thehigh-reflectivity domain extends between approximately 60 to 90 degreesincidence (in relation to the surface normal), and the required in-planecollimation is within 30 degrees of any plane parallel to the emissivesurface.

Conveniently, injection which eliminates imaging of the sources byexclusively populating the first angular distribution of radiantintensity also minimizes the gradual decay of emission between theilluminated edge(s) and points further removed from these within theoutput surface.

In some embodiments, the backlights can include a conversion structurepositioned within the cavity to convert at least a portion of light inthe cavity having the first angular distribution into light having thesecond angular distribution and at least a portion of light in thecavity having the second angular distribution into light having thefirst angular distribution. The conversion structure can be positionedproximate the front reflector, back reflector, or between the frontreflector and back reflector. Any suitable material or materials can beused to form the converting structure. In some embodiments, theconverting structure can include a semi-specular reflective material orstructure, e.g., the semi-specular reflective materials describedherein. For example, the conversion structure can be a back reflectorthat includes a bead-coated ESR. At least a portion of light within thefirst angular distribution that is propagating in the cavity isconverted to the second angular distribution after interacting with thesemi-specular back reflector.

EXAMPLES

The following Examples include various sizes of edge-lit and direct-litbacklight configurations. The tested backlights included different filmsfor both the front and back reflectors as is shown in Table 2 below.

Front and Back Reflector Films

The following is a description of the front and back reflector filmsused in the Examples:

89% R Asymmetric Reflective Film (ARF-89). This asymmetric reflectivefilm included 264 alternating microlayers of birefringent 90/10 coPENand non-birefringent PMMA. The 264 alternating microlayers were arrangedin a sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across a bandwidth from approximately 400 nm to 900 nmwavelength for one polarization axis, and a weaker reflection resonancefor the orthogonal axis. Five micron thick skin layers of 90/10 coPENwere disposed on the outside surfaces of the coherent alternatingmicrolayer stack. The overall thickness of the film, including thealternating microlayers, the PBLs and the skin layers, was approximately40 μm. This film was manufactured using the techniques described herein.

The birefringent refractive index values (measured at 633 nm) for the90/10 coPEN layers were nx1=1.785, ny1=1.685, nz1=1.518, and the indicesfor the PMMA layers were nx2=ny2=nz2=1.494.

ARF-89 had an average on-axis reflectivity of 89% in the pass axis, anaverage on-axis reflectivity of 98% in the block axis, and ahemispherical reflectivity of 92.5%.

86% R Asymmetric Reflective Film (ARF-86). This asymmetric reflectivefilm included 264 alternating microlayers of birefringent 90/10 coPENand non-birefringent PMMA. The 264 alternating microlayers were arrangedin a sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across a bandwidth from approximately 410 nm to 890 nmwavelength for one polarization axis, and a weaker reflection resonancefor the orthogonal axis. Twenty-five micron thick skin layers of SA115were disposed on the outside surfaces of the coherent alteringmicrolayer stack. The overall thickness of the film, including thealternating microlayers, the PBLs and the skin layers, was approximately75 μm. This film was manufactured using the techniques described herein.

The birefringent refractive index values (measured at 633 nm) for the90/10 coPEN layers were nx1=1.805, ny1=1.665, nz1=1.505, and the indicesfor the PMMA layers were nx2=ny2=nz2=1.494.

ARF-86 had an average on-axis reflectivity of 86% in the pass axis, anaverage on-axis reflectivity of 98% in the block axis, and ahemispherical reflectivity of 92.1%.

84% R Asymmetric Reflective Film (ARF-84). This asymmetric reflectivefilm included 264 alternating microlayers of birefringent 90/10 coPENmaterial and non-birefringent PMMA material. The 264 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across a bandwidth fromapproximately 400 nm to 900 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of 90/10 coPEN were disposed on the outside surfaces of thecoherent alternating microlayer stack. The overall thickness of ARF-84,including the alternating microlayers, the PBLs and the skin layers, wasapproximately 40 μm. This film was manufactured using the techniquesdescribed herein.

The birefringent refractive index values (measured at 633 nm) for thealternating microlayers of 90/10 coPEN were nx1=1.785, ny1=1.685, andnz1=1.518; and the indices for the microlayers of PMMA werenx2=ny2=nz2=1.494.

ARF-84 had an average on-axis reflectivity of 83.7% in the pass axis, anaverage on-axis reflectivity of 97.1% in the block axis, and ahemispherical reflectivity of 88.5%.

68% R Asymmetric Reflective Film (ARF-68). This asymmetric reflectivefilm included 274 alternating microlayers of birefringent 90/10 coPENmaterial and non-birefringent PMMA material. The 274 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across bandwidth fromapproximately 400 nm to 970 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of a blend of 75% SA115 (available from Eastman Chemical Company)and 25% DP2554 were disposed on the outside surfaces of the coherentalternating microlayer stack. The overall thickness of the asymmetricreflective film, including the alternating microlayers, the PBLs and theskin layers, was approximately 50 μm. This film was manufactured usingthe techniques described herein.

The birefringent refractive index values for the alternating microlayersof 90/10 coPEN and of PMMA material were measured at 633 nm. The indicesfor the coPEN microlayers were nx1=1.820, ny1=1.615, and nz1=1.505. Theindex of refraction for the PMMA microlayers were nx2=ny2=nz2=1.494.

ARF-68 had an average on-axis reflectivity of 68.4% in the pass axis, anaverage on-axis reflectivity of 99.5% in the block axis, and ahemispherical reflectivity of 83.2%.

37% R Asymmetric Reflective Film (ARF-37). This asymmetric reflectivefilm included 274 alternating microlayers of birefringent 90/10 coPENand non-birefringent blend of CoPET-F and DP29341. The 274 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across a bandwidth fromapproximately 420 nm to 850 nm for one polarization axis, and a weakerreflection resonance for the orthogonal axis. Five micron thick skinlayers of coPEN 55/45/HD were disposed on the outside surfaces of thecoherent alternating microlayer stack. The overall thickness of ARF-37,including the alternating microlayers, the PBLs and the skin layers, isapproximately 50 μm. This film was manufactured using the techniquesdescribed herein.

The measured birefringent refractive index values (measured at 633 nm)for the alternating microlayers of 90/10 coPEN were nx1=1.820,ny1=1.615, and nz1=1.505, and the indices for the layers ofcoPET-F+DP29341 were nx2=ny2=nz2=1.542.

ARF-37 had an average on-axis reflectivity of 38.1% in the pass axis, anaverage on-axis reflectivity of 99.0% in the block axis, and ahemispherical reflectivity of 67.6%.

2 Layer Laminate of Asymmetric Reflective Film (2×ARF). This asymmetricreflective film included two asymmetric reflective films bonded togetherusing one thick optical adhesive layer to form a laminate. Each filmincluded 274 alternating microlayers of birefringent 90/10 coPEN andnon-birefringent PET-G. The 274 alternating microlayers were arranged ina sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across a bandwidth from approximately 410 nm to 940 nm for onepolarization axis, and a weaker reflection resonance for the orthogonalaxis. There were no skin layers on the individual multilayer opticalfilms. Each film was manufactured using the techniques described herein.The overall thickness of 2×ARF, including the alternating microlayers,PBLs and adhesive layers, was approximately 100 μm. The birefringentrefractive index values (measured at 633 nm) for the alternatingmicrolayers of 90/10 coPEN were nx1=1.830, ny1=1.620, and nz1=1.500, andthe indices for the microlayers of PET-G were nx2=ny2=nz2=1.563.

2×ARF had an average on-axis reflectivity of 36% in the pass axis, and ahemispherical reflectivity of 75.4%.

3 Layer Laminate of Asymmetric Reflective Film (3×ARF). This asymmetricreflective film included three asymmetric reflective films bondedtogether using two thick optical adhesive layers to form a laminate.Each film included 274 alternating microlayers of birefringent 90/10coPEN and non-birefringent of PET-G. The 274 alternating microlayerswere arranged in a sequence of ¼ wave layer pairs, where the thicknessgradient of the layers was designed to provide a strong reflectionresonance broadly and uniformly across a bandwidth from approximately410 nm to 940 nm for one polarization axis, and a weaker reflectionresonance for the orthogonal axis. There were no skin layers on theindividual multilayer optical films. Each film was manufactured usingthe techniques described herein. The overall thickness of 3×ARF,including the alternating microlayers, PBLs and adhesive layers, wasapproximately 150 μm. The birefringent refractive index values (measuredat 633 nm) for the alternating microlayers of 90/10 coPEN werenx1=1.830, ny1=1.620, and nz1=1.500, and the indices for the microlayersof PET-G were nx2=ny2=nz2=1.563.

3×ARF had an average on-axis reflectivity of 48% in the pass axis, and ahemispherical reflectivity of 75.4%.

4 Layer Laminate of Asymmetric Reflective Film (4×ARF). This asymmetricreflective film included four asymmetric reflective films bondedtogether using three thick optical adhesive layers to form a laminate.Each film included 274 alternating microlayers of birefringent 90/10coPEN and non-birefringent of PET-G. The 274 alternating microlayerswere arranged in a sequence of ¼ wave layer pairs, where the thicknessgradient of the layers was designed to provide a strong reflectionresonance broadly and uniformly across a bandwidth from approximately410 nm to 940 nm for one polarization axis, and a weaker reflectionresonance for the orthogonal axis. There were no skin layers on theindividual multilayer optical films. Each film was manufactured usingthe techniques described herein. The overall thickness of 4×ARF,including the alternating microlayers, PBLs and adhesive layers, wasapproximately 200 μm.

The measured birefringent refractive index values (measured at 633 nm)for the alternating microlayers of 90/10 coPEN were nx1=1.830,ny1=1.620, and nz1=1.500, and the indices for the microlayers of PET-Gwere nx2=ny2=nz2=1.563.

4×ARF had an average on-axis reflectivity of 55.6% in the pass axis, anda hemispherical reflectivity of 79.2%.

5 Layer Laminate of Asymmetric Reflective Film (5×ARF). This multilayeroptical film is included four thick optical adhesive layers used to bondfive sheets of asymmetric reflective film in a laminate body. Each filmincluded 274 alternating microlayers of birefringent 90/10 coPEN andnon-birefringent of PET-G. The 274 alternating microlayers are arrangedin a sequence of ¼ wave layer pairs, where the thickness gradient of thelayers was designed to provide a strong reflection resonance broadly anduniformly across bandwidth from approximated 410 nm to 940 nm wavelengthfor one polarization axis, and a weaker reflection resonance for theorthogonal axis. There were no skin layers on the individual multilayeroptical films. The overall thickness of 5×ARF, including the alternatingmicrolayers, PBLs and adhesive layers, was approximately 260 μm. Themeasured (at 633 nm) birefringent refractive index values for thealternating microlayers of 90/10 coPEN material were nx1=1.830,ny1=1.620, and nz1=1.500, and the indices of the PET-G material werenx2=ny2=nz2=1.563.

In the following Examples, 5×ARF was used with an Opalus BS-702 beadedgain diffuser (available from Keiwa Corp., Japan) laminated to the sideof the surface of the 5×ARF that faced the back reflector, such that thebeads (i.e., microspheres) of the gain diffuser faced toward the backreflector.

5×ARF laminated to the beaded gain diffuser had an average on-axisreflectivity of 61.7% in the pass axis, and a hemispherical reflectivityof 81.1%.

Bead-coated ESR (BESR). This optical film included a plurality ofoptical elements coated onto an ESR film. The coating process includeddispersing a size distribution with geometric mean diameter of ˜18 μm,of small PMMA beads (MBX-20, available from Sekisui, Japan) into asolution of Iragacure 142437-73-01, IPA, and Cognis Photomer 6010. Thesolution was metered into a coater, and subsequently UV cured, producinga dried coating thickness of approximately 40 μm. At this thickness, thedispersion of PMMA beads created a partial hemispheric surfacestructure, randomly distributed spatially. The average radius ofprotrusion of the PMMA beads above the mean surface was estimated to beapproximately 60% of the average bead radius. The dried matrix wasformulated to have approximately the same refractive index as the PMMAbeads, minimizing the bulk scattering within the coating. BESR had ahemispherical reflectivity of 98.0%.

ESR. Vikuiti™ Enhanced Specular Reflector multilayer polymeric filmavailable from 3M Company. ESR had a hemispherical reflectivity of99.4%.

BGD. Unless otherwise specified, some of the following Examples includeda Opalus BS-702 beaded gain diffuser (available from Keiwa Corp.).

2×TIPS. A porous polypropylene film having a high reflectivity and canbe made using thermally induced phase separation as described, e.g., inU.S. Pat. No. 5,976,686 (Kaytor et al.). Two sheets of TIPS werelaminated together using an optical adhesive to form a laminate. TheLambertian diffuse reflector had an average hemispherical reflectivityof 97.5%.

DBEF. Multilayer reflective polarizing film available from 3M Company.DBEF had a hemispheric reflectivity of 50.8%.

APF. Multilayer reflective polarizing film available from 3M Company.APF had a hemispheric reflectivity of 51.0%.

LEF. Light Enhancement Film 3635-100, available from 3M Company. Thisfilm is diffusely reflective. LEF had a hemispheric reflectivity of 94%.

MCPET. Microcellular PET reflective sheeting, available from FurukawaAmerica, Inc. (Peachtree City, Ga.). MCPET is diffusely reflective.

The following table indicates which front and back reflector films wereused for each Example:

TABLE 2 Example Front Reflector Back Reflector Comparative Example 1 BGDESR  1 ARF-89 Brushed Aluminum  2 ARF-89 ESR  3 ARF-89 BESR  4 ARF-89LEF  5 ARF-89/BGD ESR  6a ESR BESR  6b ARF-89 BESR  6c ARF-84 BESR  6dARF-68 BESR  6e ARF-37 BESR  6f APF BESR  7 ARF-89 BESR  8 ARF-89 BESR 9 ARF-89 BESR 10a ARF-68/BGD ESR 10b ARF-68/BGD ESR 11a ARF-68/BGD ESR11b ARF-68/BGD ESR 12a ESR BESR 12b ARF-89 BESR 12c ARF-84 BESR 12dARF-68 BESR 12e ARF-37 BESR 12f APF BESR Comparative Example 2 APF/BGDESR 13 5xARF/BGF ESR 14 5xARF/BGF ESR 15 ARF-68 BESR Comparative Example3 APF BESR 16 ARF-89 BESR 17 ARF-68 BESR 18 4xARF BESR 19 4xARF 2xTIPS20 ARF-89 BESR 21 ARF-89 BESR Comparative Example 4 APF/BGD ESR 22ARF-68/BGD ESR 23 ARF-68/BGD ESR 24 ARF-68/BGD ESR/BGD ComparativeExample 5 Diffuser Plate 2xTIPS 25 Bead Coated ARF-84 2xTIPS 26ARF-84/BGD 2xTIPS Comparative Example 6 Diffuser Plate 2xTIPSComparative Example 7 Diffuser Plate/DBEF 2xTIPS 27 DiffuserPlate/ARF-37 2xTIPS 28 Diffuser Plate/3xARF 2xTIPS Comparative Example 8DP/BDG/BEF/DBEF 2xTIPS 29 BGD/BEF/ARF-68 ESR 30 (2) ARF-84/BGD 2xTIPS 31DP/2xARF MCPETMeasurement Systems

The luminance and uniformity of the examples were measured using one oftwo models of colorimetric cameras (either model PM 1613F-1 or modelPM-9913E, both from Radiant Imaging, Inc.). These two models ofcolorimetric camera produce data that are nearly identical when properlycalibrated. Unless otherwise noted, the camera was fitted with a 105 mmlens and the internal ND2 neutral density filter was selected. Thesoftware supplied by Radiant Imaging was used to calibrate the cameraand take the measurements. Color and luminance calibration was done withthe aid of a spot radiometer (model PR650 from Photo Research, Inc.).The test bed was placed in the vertical, orientation, 5 m in front ofthe camera. The test bed was aligned to the camera such that the axis ofthe camera lens was normal to the output area and aimed approximately atthe center of the test system. The camera software was set to recordonly the display portion of the image using the clipping feature.Exposure time was set automatically by the software to avoidoverexposure of the images.

Measurements were carried out by configuring the test bed with the filmsto be tested and then using the colorimetric camera to take pictures ofthe test system. The average luminance, average color, luminanceuniformity and color uniformity were calculated from the measured imagesusing custom data analysis programs.

Unless otherwise noted, the data was measured through an absorbingpolarizer (HLC2-5618S from Sanritz) laminated to an acrylic plate. Thepass axis of the absorbing polarizer was oriented parallel with the passaxis of the front reflector film.

Data collected using the colorimetric camera were analyzed to determinethe average luminance, average color, luminance uniformity and coloruniformity. In the following examples, the average luminance value wascalculated by summing the luminance value of every pixel in an image anddividing by the total number of pixels in the recorded image. Since theimage data was recorded using a colorimetric camera, this is an on-axisluminance value. Similarly, the average color (expressed in the colorcoordinates on the CIE 1976 Uniform Chromaticity Scale and hereinreferred to as u′ and v′) was obtained by summing u′ or v′ over all ofthe pixels in the image and dividing by the total number of pixels inthe recorded image.

Luminance and color uniformity was determined according to the VideoElectronics Standards Association's Flat Panel Display MeasurementsStandard, v. 2.0 (published Jun. 1, 2001) standard 306-1 SampledUniformity and Color of White. Nine sampling points were used as definedin the Standard. The luminance or color at each sample point wasdetermined from the recorded image by averaging the luminance or u′ andv′ values of the pixels falling within an approximately circular regionaround the sample point location. The approximately circular region hada diameter of 3% of the diagonal of the image.

The VESA 9 pt luminance uniformity reported herein was determined fromthe 9 sample points as

${V\; E\; S\; A\mspace{14mu} 9\mspace{14mu}{pt}\mspace{14mu}{Luminance}\mspace{14mu}{Uniformity}} = \frac{L_{m\; i\; n}}{L_{m\;{ax}}}$

where L_(min) is the minimum value of the luminance of the 9 points andL_(max) is the maximum value of the luminance of the 9 points. Highervalues of VESA 9 pt luminance uniformity indicate systems that are moreuniform.

The VESA 9 pt color nonuniformity was determined as the largest value ofthe color difference between any two pairs of the 9 sampled points. Thecolor difference Δu′v′ isΔu′v′=√{square root over ((u′ ₁ −u′ ₂)²+(v′ ₁ −v′ ₂)²)}{square root over((u′ ₁ −u′ ₂)²+(v′ ₁ −v′ ₂)²)}where the subscripts 1 and 2 denote the two points being compared. Lowervalues of VESA 9 pt color nonuniformity indicate systems that are moreuniform.

Hemispherical reflectivity (R_(hemi)) for several front and backreflectors was measured by using the following technique. A commercialsix inch integrating sphere manufactured by Labsphere and made ofSpectralon, with three mutually orthogonal ports, was used to illuminatesamples and to determine hemispherical reflectivity. A stabilized lightsource illuminated the sphere through one port. A spot spectroradiometer(model PR650, available from Photo Research, Inc., Chatsworth, Calif.)was used to measure the sphere internal wall radiance through a secondport. The sample was placed on the third port. Calibration of theintegrating sphere wall radiance was done by using a known reflectancestandard placed on the third port; sphere-wall radiance was measuredwith and without the calibration standard. R_(hemi) was measured byplacing a sample on third port. R_(hemi) was then calculated by takingthe ratio of the sphere wall radiance with and without sample, andemploying a simple integrating sphere brightness-gain algorithm. Thismeasurement of R_(hemi) can be germane to recycling backlight cavityperformance in that it is the all-angle input, all-angle outputreflection, measured in a way much like that which occurs in an actualrecycling cavity.

885 mm×498 mm Edge-lit Backlight (40″ Backlight Cavity)

The following examples were tested in a custom LED backlight test bed.The test bed was designed to simulate an LED-based area backlight for a1016 mm (40″) diagonal, 16:9 aspect-ratio, LCD panel. The hollow testbed backlight cavity had a bent sheet metal shell forming side walls andrear walls, with the long axis of the frame being placed horizontally.Both the top and bottom of the frame were open to allow for insertion ofLED light engines. The internal cavity depth was 30 mm, with anapproximately 5 mm bend radius at the back wall to side wall interface.The sheet metal used was commercial grade brushed aluminum with athickness of 0.75 mm.

As is further described below, various front reflector films were eachattached to 2.5 mm thick clear PMMA plates (Cyro Acrylite FF availablefrom Cyro Corp., Rockaway, N.J.) by either static cling or throughlamination with 3M OPT1™ optical transfer adhesive (available from 3MCompany). The plates were attached to the hollow backlight cavity suchthat the front reflector faced into the cavity and the PMMA plate formedthe outermost emissive surface of the test bed. The outer surface of theplate serves as the output surface for the test bed (i.e., the outputsurface of the backlight).

Five LED bars (“engines”) were affixed to the bottom edge of thebacklight frame. The bars along the bottom edge were arranged in asingle row spanning the width of the backplane. Each bar had 5 red, 5blue, and 10 green Lambertian Luxeon™ 3 LEDs (available from Lumileds,San Jose, Calif.) arranged in a repeating green-red-blue-green patternin a single line on a standard flexible printed-circuit. Thecenter-to-center spacing between LEDs on a single bar was about 8.5 mm.The center-to-center spacing between LEDs at the interface betweenengines was about 16 mm. The total length of a single light engine was174 mm. The center to center pitch between light engines wasapproximately 180 mm. Each flexible printed circuit was thermallymounted to a heat sink using thermally conductive adhesive. The engineswere run at approximately 55° C. at the base of the heat sink. Each heatsink included a fan.

The LEDs included the LUXEON III EMITTER RED LAMB (LXHL-PD09 LML),LUXEON III EMITTER GREEN LAMB (LXHL-PM09 LML), and LUXEON III EMITTERR-BLUE LAMB (LXHL-PR09 LML) (available from Lumileds, San Jose, Calif.).

On a single bar, the green, red, and blue LEDs were electricallyconnected in series by color so that the output of each color could bevaried independently to allow for adjusting the color balance of thetest bed. Custom 4-channel power driver boards were used to drive theLEDs. One power supply channel provided the drive current to the redLEDs, one channel provided current to the blue LEDs, and two channelsprovided current to the green LEDs each channel driving 5 of the greenLEDs. After stabilization, the LED currents were adjusted in each engineto result in a D65 white point for the mixed light.

Five different LED bars were also affixed to the top edge of the hollowtest bed. The bars along the top edge were arranged in a single rowspanning the width of the backplane. Each of these bars had 3 red, 3blue, and 12 green Lambertian Luxeon™ 3 LEDs (available from Lumileds)arranged in a green-green-red-blue-green-green pattern in a single lineon a standard flexible printed-circuit board. The center-to-centerspacing between LEDs on a single bar was about 8.5 mm. The spacingbetween LEDs at the interface between engines was about 31 mm. The totallength of a single light engine was 157 mm. The center to center pitchbetween light engines was approximately 180 mm.

On a single bar, the green, red, and blue LEDs were electricallyconnected in series by color so that the output of each color could bevaried independently to allow for adjusting the color balance of thetest bed. Custom 4-channel power driver boards were used to drive theLEDs. One power supply channel provided the drive current to the redLEDs, one channel provided current to the blue LEDs, and two channelsprovided current to the green LEDs each channel driving 6 of the greenLEDs. The LED currents were adjusted in each engine to result in a D65white point for the mixed light after the LCD panel, using an asymmetricreflective film having an average on-axis transmission of 11% to sealthe cavity, and having bead-coated ESR as the back reflector. Theengines were run at an approximately 55° C. temperature at the base ofthe heat sink. The approximate currents at which each light engine wasrun was red at 1.1 A, blue at 1.1 A, and each green at 0.44 A.

An aluminum wedge reflector was used to direct the light from each LEDlight engine into the hollow recycling cavity. The wedge included ataper that tapered from 7.8 mm at the base to 15.6 mm at the entrance tothe backlight cavity. The length of the wedge was 47.3 mm. The centralaxis of the wedge was slightly tilted toward the back reflector.

The proximal edge of the wedge plate had holes to allow the LED lensesto extend through the plate. When mounted, the top surface of the platewas aligned with the bottom of the LED lenses. ESR was laminated to theinside of the wedge. Thus mounted, the film layer was substantially flaton each face of the wedge and acted as a focusing reflector directingthe LED light into the cavity.

The performance of the test bed was measured using a colorimetric camera(model PM 1613F-1 available from Radiant Imaging, Inc., Duvall, Wash.).The camera was fitted with a 105 mm lens (Sigma EX 105 mm 1:2.8D DGMacro) and a ND2 neutral density filter. Unless specified, an absorptivepolarizer (Sanritz 5516) was used in front of the camera lens with thepass axis of the polarizer aligned to match the pass axis of the frontfilms. The software supplied by Radiant Imaging was used to calibratethe camera and take the measurements. Color and luminance calibrationwas done with the aid of a spot spectroradiometer (model PR650 availablefrom Photo Research, Inc., Chatsworth, Calif. or a Minolta CS-100 fromKonica Minolta Sensing Americas, Inc., Ramsey, N.J.). The test bed wasplaced in the vertical orientation, 5 meters in front of the camera. Thetest bed was aligned to the camera such that the axis of the camera lenswas normal to the front plate and aimed approximately at the center ofthe test bed.

Measurement data was analyzed according to the Video ElectronicsStandards Association Flat Panel Display Measurements Standard Version2.0. For cases with LCD backlight, section 306-1 “Sampled Uniformity &Color of White” was used. When no LCD was present on the backlight, avariant on the section 306-1 standard measurement coordinates andprocedures were used, wherein the LCD panel was omitted.

Backlight constructions were measured by mounting the appropriate films(back reflector and front reflector) in the test bed and selecting whichLED banks were turned on.

The LEDs were turned on and warmed up for at least 30 minutes prior torecording any measurements. Measurements were carried out by configuringthe test bed with the films to be tested, and then using thecolorimetric camera to take pictures of the test bed. The results wereinspected visually and analyzed for properties such as total luminance,luminance uniformity, and color uniformity across the surface of thefront plate.

Comparative Example 1 BGD Front Reflector

The 40″ backlight cavity was configured having an Opalus BS-702 gaindiffuser (available from Keiwa Corp., Tokyo, Japan) as the frontreflector and ESR as the back reflector. The beaded gain diffuser wasoriented so that the structured surface was external to the hollowcavity. The display was lit with only the bottom banks of LEDs.

The appearance of the output surface, i.e., the top of the diffusersheet, was highly non-uniform. Horizontal bright and dark bandscorresponding to the output of the injection wedge were observed about 3to 8 inches from the injection edge. The overall brightness across thedisplay decreased substantially with increasing distance from theinjection wedge. A colorimetric image of the backlight was recordedusing the PM 1613F-1 colorimetric camera as described above. The averageluminance was 704 cd/m² and the VESA 9 pt luminance uniformity was 37%.This display configuration is of limited usefulness because of thebanding artifacts and the rapid brightness drop-off.

Example 1 ARF-89 Front Reflector and Brushed Aluminum Back Reflector

In this example, the 40″ recycling cavity was formed from a frontreflector that included ARF-89. The back reflector included brushedaluminum. The cavity depth was 30 mm. The sidewalls were also brushedaluminum. Only the bottom banks of LEDs were lit.

In appearance, this example was dark and demonstrated substantialvertical streaking of the image, with each streak imaging eachindividual LED. Individual red, green, and blue LEDs could bedistinguished. A colorimetric image of the backlight was recorded usingthe PM 1613F-1 colorimetric camera as described above. The averageluminance was 127 cd/m² and the VESA 9 pt luminance uniformity was 65%.This display configuration would not be suitable for backlightingapplications because of the color streaking and the brightnessnonuniformity.

Example 2 ARF-89 Front Reflector and ESR Back Reflector

In this example, the 40″ recycling cavity was formed from a frontreflector formed from ARF-89 and a back reflector formed from ESR. Nodiffuser or diffuser film was included in the cavity or on the outsideof the emitting surface. The cavity depth was 30 mm. The sidewalls werealso ESR. Only the bottom banks of LEDs were lit.

A colorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 382cd/m² and the VESA 9 pt luminance uniformity was 17%. In appearance,this example demonstrated substantially the “hall of mirrors” effect,where the LEDs and the edges of the display were repeatedly imaged. Thisdisplay configuration would not be suitable for backlightingapplications because of the numerous artifacts.

Example 3 ARF-89 Front Reflector and BESR Back Reflector

The 40″ recycling cavity was formed from a front reflector that includedARF-89 and a back reflector that included BESR. The cavity depth was 30mm. The sidewalls were also covered with BESR. Only the bottom banks ofLEDs were lit.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. The bright banding seen inComparative Example 1 was greatly diminished, nor were the color streaksseen that were observed in Example 1. The overall brightness across thedisplay decreased by about a factor of 2 with increasing distance fromthe injection wedge; this decrease was gradual and smooth in appearance.A colorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 1492cd/m² and the VESA 9 pt luminance uniformity was 55%. This backlightconfiguration could be suitable for some lower performance backlightapplications because of the smoothness of the decrease in brightness.

Example 4 ARF-89 Front Reflector and LEF Back Reflector

In this example, the 40″ recycling cavity was formed from a frontreflector that included ARF-89 and a back reflector that included LEF.The cavity depth was 30 mm. Only the bottom banks of LEDs were lit.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. The bright banding wasgreatly diminished. The overall brightness across the display smoothlydecreased by about a factor of 2 with increasing distance from theinjection wedge. A colorimetric image of the backlight was recordedusing the PM 1613F-1 colorimetric camera as described above. The averageluminance was 1299 cd/m² and the VESA 9 pt luminance uniformity was 46%.This backlight configuration could be suitable for some lowerperformance backlighting applications because of the smoothness of thedecrease in brightness.

Example 5 ARF-89/BGD Front Reflector, ESR Back Reflector

The 40″ recycling cavity was formed from a front reflector that includedARF-89 and a back reflector including ESR. A beaded gain diffuser film(Opalus BS-702) was included in the cavity with the beaded surfacefacing into the cavity and the back of the film contacting the frontreflector. The cavity depth was 30 mm. The sidewalls were also ESR. Onlythe bottom banks of LEDs were lit.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. The bright banding wasgreatly diminished. The overall brightness across the display decreasedwith increasing distance from the injection wedge. A colorimetric imageof the backlight was recorded using the PM 1613F-1 colorimetric cameraas described above. The average luminance was 1562 cd/m² and the VESA 9pt luminance uniformity was 75%. This backlight configuration could besuitable for some medium to high performance backlighting applicationsbecause of the smoothness of the decrease in brightness.

Examples 6a-f Various Front Reflectors and BESR Back Reflector

In this example, the 40″ recycling cavity was formed from a backreflector that included BESR and several different front reflectors thatincluded the asymmetric reflective films shown in the following Table 3.The cavity depth was 30 mm. The sidewalls were also covered with BESR.Only the bottom banks of LEDs were lit.

TABLE 3 Front Reflectors VESA 9 pt. Average Luminance Luminance ExampleFront Reflector Uniformity (%) (cd/m²) 6a ESR 46 52 6b ARF-89 55 1492 6cARF-84 49 1872 6d ARF-68 33 2474 6e ARF-37 23 2419 6f APF 15 1921

In appearance, examples 6a-e demonstrated substantial uniformityimprovement over the Comparative Examples. The following are visualobservations for each of these examples:

-   -   6a. Negligible brightness banding, but very dim and colors were        highly nonuniform, ranging from magenta at the bottom to blue at        the top of the display.    -   6b. Slight amount of horizontal brightness banding observed.        Substantially higher brightness than Example 6a. A slight color        shift was observed between the bottom of the display and the        top. This could make an acceptable backlight for lower        performance applications because of the gradual brightness        change.    -   6c. Somewhat noticeable amount of banding observed. This could        make an acceptable backlight for lower performance applications        because of the smoothness of the brightness change.    -   6d. Noticeable banding observed. This could make an acceptable        backlight for very low performance applications because of the        smoothness of the brightness change.    -   6e. Very noticeable amount of banding observed. Substantially        higher brightness than Example 6a. This is unlikely to make a        suitable backlight for any but the least demanding applications.    -   6f. Very noticeable amount of banding observed. Substantially        higher brightness than Example 6a. This could make an acceptable        backlight for lower performance applications because of the        smoothness of the brightness change.

Example 7 ARF-89 Front Reflector and BESR Back Reflector

The 40″ recycling cavity was set up in similar fashion to Example 3. Inthis case, however, both the bottom banks and top banks of LEDs werelit.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. The bright banding wasgreatly diminished. The uniformity was improved over Example 3 as well.A colorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 2764cd/m² and the VESA 9 pt luminance uniformity was 84%. This backlightwould be suitable for high performance applications.

Example 8 ARF-89 Front Reflector and BESR Back Reflector

In this example, the 40″ recycling cavity was set up in similar fashionto Example 7. In this case, however, the Sanritz 5516 absorptivepolarizer was removed from the image path.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. The bright banding wasgreatly diminished. The uniformity was similar to that of Example 7. Acolorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 3462cd/m² and the VESA 9 pt luminance uniformity was 84%. The averagebrightness was only 25% greater than Example 7; however, the brightnessof the backlight of Example 8 would be expected to be 100% brighter thanExample 7 if the backlight of Example 8 were unpolarized. That Example 8exhibits only 25% greater brightness demonstrates that the backlightselectively polarizes light.

Example 9 ARF-89 Front Reflector and BESR Back Reflector

The 40″ recycling cavity was set up in similar fashion to Example 7. Inthis case, however, a single set of 20 LEDs (1 engine) from the centerbottom were turned off.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Examples. Visually, the region aroundthe turned-off engine was somewhat dimmer, but not objectionable. Acolorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 2677cd/m² and the VESA 9 pt luminance uniformity was 69%. This backlightwould be suitable for many conventional applications.

Examples 10a-b ARF-89/BGD Front Reflector and ESR Back Reflector

In Example 10a, the 40″ recycling cavity was formed from a frontreflector that included ARF-89 and a back reflector that included ESR. Abeaded gain diffuser film (Opalus BS-702) was included in the cavitywith the beaded surface facing into the cavity and the back of the filmcontacting the front reflector. The cavity depth was 30 mm. Thesidewalls were also ESR. Both top and bottom banks of LEDs were lit.

A colorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 4605cd/m² and the VESA 9 pt luminance uniformity was 86%. This backlightwould be suitable for high performance applications. No visual artifactswere observed.

For Example 10b, a 40″ diagonal LCD panel was placed in front of theemitting surface of the backlight of Example 10a. The LCD panel was froma Samsung model LNR-408D television (available from Samsung ElectronicsAmerica, Inc., Ridgefield Park, N.J.), which, according to Samsung'sliterature, uses Samsung's Patterned Vertical Alignment (PVA)technology. The original LCD television was disassembled and the LCDpanel and, necessary drive electronics were extracted for independentuse over the LED backlight of Example 10A.

The LCD panel was turned on and driven in the fully on white state. Acolorimetric image of the display was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 567cd/m² and the VESA 9 pt luminance uniformity was 84%. This backlightwould be suitable for high performance applications. Again, no obviousvisual artifacts were detected.

Examples 11a-b ARF-68/BGD Front Reflector and ESR Back Reflector withCCFL Light Sources

For Example 11a, a 19″ diagonal recycling cavity was formed from a frontreflector that included ARF-68 and a back reflector that included ESR. Abeaded gain diffuser film (Opalus BS-702) was included in the cavitywith the beaded surface facing into the cavity and the back of the filmcontacting the front reflector. The cavity depth was 0.4 inches (10 mm).The sidewalls were also ESR. Both top and bottom pairs of CCFL bulbswere lit.

A colorimetric image of the backlight was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 1394cd/m² and the VESA 9 pt luminance uniformity was 39%. In this case,visual artifacts due to film nonuniformities were seen. These consistedof diagonal striations across the display and a greenish hue in one halfof the display. As a consequence of these defects and the brightnessnonuniformity, this would not make a suitable high performance display.However, it does demonstrate that the films can be used with CCFLbacklights.

For Example 11b, a 19″ diagonal LCD panel was placed in front of theemitting surface of the backlight of Example 11a. The LCD panel was froma Samsung model 940BW (available from Samsung Electronics America,Inc.), which, according to their literature, is a TFT-LCD. The originalLCD display was disassembled and the LCD panel and necessary driveelectronics were extracted for independent use over the LED backlight ofExample 11a.

The LCD panel was turned on and driven in the fully on white state. Acolorimetric image of the display was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 217cd/m² and the VESA 9 pt luminance uniformity was 38%. The backlightlooked very much like that of Example 11a; however, the backlightappeared to be more dim than that of Example 11a.

Examples 12a-f Examples 6a-f with Two Sided Illumination

Examples 6a-f were tested with both the top and bottom banks of LEDslit. Table 4 is a summary of the resulting data.

TABLE 4 VESA 9 pt Average Luminance Luminance Example Front ReflectorUniformity (%) (cd/m²) 12a ESR 56 92 12b ARF-89 84 3402 12c ARF-84 844361 12d ARF-68 62 4340 12e ARF-37 50 3385 12f APF 38 1010

In appearance, 12a-e demonstrated substantial uniformity improvementover the Comparative Examples. The following are visual observations foreach of these examples:

-   -   12a. Negligible brightness banding, but very dim and colors were        highly nonuniform, ranging from magenta at the bottom to blue at        the top of the display.    -   12b. No horizontal brightness banding observed. Example        exhibited substantially higher brightness than Example 12a. A        slight color shift was observed between the bottom of the        display and the top. This could make an acceptable backlight for        higher performance applications because of the gradual        brightness change and its overall brightness uniformity.    -   12c. No horizontal brightness banding observed. Colors appear        uniform. This could make an acceptable backlight for higher        performance applications because of the gradual brightness        change and its overall brightness uniformity.    -   12d. Slightly noticeable horizontal brightness banding observed.        This could make an acceptable backlight for medium performance        applications because of the gradual brightness change and its        overall brightness uniformity.    -   12e. Some horizontal brightness banding observed. This could        make an acceptable backlight for very low performance        applications because of the smoothness of the brightness change.    -   12f. Noticeable amount of horizontal brightness banding        observed. This could make an acceptable backlight for very low        performance applications because of the smoothness of the        brightness change.

Comparative Example 2 APF/BGD Front Reflector and ESR Back Reflector

In this example, a 40″ backlight was formed in a manner similar to thatof Example 5. The backlight included a front reflector that included APFand a back reflector that included ESR. A beaded gain diffuser (OpalusBS-702) was included in the cavity with the structured surface facinginto the cavity and the back of the film contacting the APF. The cavitydepth was 30 mm. The sidewalls were also ESR. Both the top and bottombanks of LEDs were lit.

In appearance, this example demonstrated substantial uniformityimprovement over the Comparative Example 1. Moderate bright banding wasstill observed. A colorimetric image of the backlight was recorded usingthe PM 1613F-1 colorimetric camera as described above. The averageluminance was 3844 cd/m² and the VESA 9 pt luminance uniformity was 78%.The color uniformity appeared to be very good. This backlight would besuitable for moderate performance backlighting applications. Thebrightness banding would likely make it unsuitable for high-performanceapplications.

Example 13 708 mm×398 mm Edge-lit Backlight

The following example was tested in a custom LED backlight test bed. Thetest bed was designed as an LED-based area backlight for a 813 mm (32″)diagonal, 16:9 aspect-ratio, LCD television. The hollow test bedbacklight cavity had a bent sheet metal shell forming side walls andrear walls, with the long axis of the frame being placed horizontally.Slots were cut into the back reflector to allow for insertion of LEDlight engines. The internal cavity depth was 19 mm, with anapproximately 5 mm bend radius at the back wall to side wall interface.The sheet metal used was commercial grade brushed aluminum with athickness of 1 mm.

The front reflectors included 5×ARF laminated to a 0.005″ (0.127 mm)thick 12% Haze PC sheet using 3M OPT1™ optical transfer adhesive(available from 3M Company). The final layer on the laminate stack wasan Opalus™ BS-702 beaded gain diffuser.

The low-birefringence, low haze, polycarbonate film used was 0.005″thick (0.127 mm) with a 12% haze level (Iupilon™ Film, Grade FE-2000M05, Mitsubishi Engineering-Plastics Corp., Tokyo, Japan). In all casesit was laminated to the front reflector with the textured surface facingin towards the adhesive.

The plates were attached to the hollow backlight cavity such that thebeaded gain diffuser faced into the hollow cavity and the polycarbonatesurface formed the outermost emissive surface of the test bed. The outersurface of the plate served as the output surface for the test bed(i.e., the output area of the backlight).

Four LED bars (“engines”) were affixed to the backside, bottom edge ofthe sheet metal shell. The bars along the bottom edge were arranged in asingle row spanning the width of the backplane. Each bar had 4 red, 2blue, 6 green, and 6 white Lambertian Cree XR-E LEDs (Cree Inc., Durham,N.C.) arranged in a GRGBGR-WWWWWW-RGBGRG pattern in a single line on astandard flexible printed-circuit. Model numbers for the LEDs were asfollows: Red (XR7090RD-L1-0001), Green (XR7090GR-L1-0001), Blue (royalblue, XR7090RY-L1-0001), and White (XREWHT-L1-0802).

The center-to-center spacing between LEDs on a single bar was about 9.5mm. The total length of a single light engine (bar) was 170 mm. Thecenter to center pitch between light engines was approximately 175 mm.

On a single bar, the red, green, blue, and white LEDs were electricallyconnected in a single series. Each bar was run at 700 mA current using acurrent regulated power supply. Each flexible printed circuit wasthermally mounted to a heat sink using thermally conductive adhesive.Fans were used to cool the heat sinks to an approximate 50 C operatingtemperature at the base of the heat sink.

An aluminum “reverse” wedge reflector was used to direct the light fromeach LED light engine into the hollow recycling cavity. (See PCT PatentApplication No. PCT/US08/64125, entitled COLLIMATING LIGHT INJECTORS FOREDGE-LIT BACKLIGHTS for a description of this reverse wedge).

The LED engines were mounted on the backside of the cavity, with theLEDs protruding somewhat into the cavity through drilled holes. The backreflector plate had holes to allow the LED lenses to extend through theplate. When mounted, the top surface of the plate was aligned with thebottom of the LED lenses.

ESR was laminated to all inner surfaces of the hollow cavity includingthe inside of the wedge and the redirector mirrors.

Thus mounted, the film layer was substantially flat on each face of thewedge and acted as a focusing reflector directing the LED light into thecavity.

The cavity was inserted behind an LCD panel from a Samsung 32 inchdiagonal TV.

The LCD panel was turned on and driven in the fully on white state. Acolorimetric image of the display was recorded using the PM 1613F-1colorimetric camera as described above. The average luminance was 432cd/m² and the VESA 9 pt luminance uniformity was 70%. The coloruniformity was very good and no bright banding was observed.

Example 14 708 mm×398 mm Edge-lit Backlight

An additional display was constructed in a similar fashion to Example13. In this example a 23 inch diagonal monitor was retrofitted with ahollow edge-lit LED light guide. Width and height dimensions wereadjusted to fit the 23 inch diagonal monitor; however, the thicknessremained 19 mm. The monitor was a Apple Cinema Display made by AppleComputer, Cupertino, Calif., and was originally backlit by CCFL bulbs.The CCFL backlight was removed as were all optical films behind the LCD.

The LEDs used in this case were OSRAM G6-SP series RGB LEDs.Configuration was GRGBGRG with 6 repeat units per engine.

The reverse wedge construction of Example 13 was used. The reverse wedgehad the same dimensions as in Example 13.

The LCD panel was turned on and driven in a fully on white state. Animage was collected with the Prometric camera. A colorimetric image ofthe backlight was recorded using the PM 1613F-1 colorimetric camera asdescribed above. The average luminance was 150 cd/m² and the VESA 9 ptluminance uniformity was 84%. The color uniformity was very good, and nobright banding was observed.

Example 15 7 Inch DVD Player with ARF-68 Front Reflector and BESR BackReflector

A 7 inch diagonal wide format Zenith DVD player was retrofitted with ahollow edge-lit light guide. A Zenith Model DVP615 Portable DVD Player(distributed by LG Electronics USA, Englewood Cliffs, N.J.) wasdisassembled and the solid light guide removed. A 4 mm deep hollowcavity was made by lining the inside of the gutted light guide metalhousing with BESR (only tacked with adhesive in the corners). Allremaining inside surfaces were covered with ESR so as to make side wallreflectors as well. No injection wedge was used.

The original 27 Lambertian white LEDs were domed using 3M photocurablesilicone PSE-002. The LED circuit board strip, which included 27Lambertian white LEDs, was adhered to the bottom edge of the metal frameusing double-stick tape. A second set of 27 LEDs was installed along theupper edge of the metal frame using parts removed from an identicalmodel DVD player. The two LED strips were wired in parallel.

Black tape was used on the top and bottom ⅓ inch (8.5 mm) of the LCDpanel to mask the LED punch-through. ARF-68 was laminated to the backsurface of the LCD panel using OPT1™ adhesive; the film was aligned suchthat the pass axis of the film was aligned with the pass axis of the LCDrear polarizer. The portable DVD player was reassembled.

The LCD panel was turned on and driven in the fully on white state. Thecolor uniformity was very good, and no bright banding was observed.

273 mm×151 mm Edge-Lit Backlights

The following examples were measured using an edge-lit backlight thatwas a rectangular box about 273 mm by 151 mm with an internal thicknessof about 25.4 mm.

The backlight was illuminated by 4 red LEDs (Luxeon III LXHL-PD09), 8green LEDs (Luxeon III LXHL-PM09), and 4 blue LEDS (Luxeon IIILXHL-PR09) for a total of 16 LEDs. The LEDs were linearly arranged on acircuit board in red-green-blue-green clusters with 8.5 mmcenter-to-center spacing between the LEDs. The LEDs were powered by acustom constant-current drive circuit. All of the LEDs of a given colorwere driven at approximately the same current. The white point of thebacklight is approximately u′=0.21 and v′=0.45. The currents used were1.4 A for red, 700 mA for green and 700 mA for blue.

The LED light source was attached to one of the short sides of the box.The LEDs were oriented with the central axis pointing into the cavity(parallel to the output face of the backlight). Light injection from theLEDs into the backlight cavity was facilitated by a wedge tapering from18.6 mm at the base to 25.3 mm at the entrance to the backlight cavity.The length of the wedge was 25.4 mm.

The inside of the distal-end side reflector (sidewall opposite the lightsources) of the backlight was covered with BESR. The inside of all theother sidewalls of the backlight, including the inside of the wedge, wascovered with ESR. The back reflector included either 2×TIPS, ESR, orBESR. These materials served as the back reflectors for the cavity andwere extended to touch the sidewalls to minimize light leaks.

Uniformity measurements were conducted using the PM-9913E colorimetriccamera. The backlight was mounted vertically at a distance of 5 m fromthe camera. A Nikon 300 mm lens at f/4 was used along with the internalND2 filter to collect the images.

Comparative Example 3 APF Front Reflector BESR Back Reflector

The front reflector included APF that was held in place against the rimof the backlight with double-stick tape. The back of the backlight wascovered with bead-coated ESR to form the back reflector.

All of the LEDs were powered and the luminance was measured. Theillumination was not uniform and appeared brighter at the end closest tothe LEDs. Near the LEDs the color also appeared to vary across thebacklight with one side appearing more blue and the other side appearingmore red relative to the center. A colorimetric image of the backlightwas recorded using the PM 9913E-1 colorimetric camera as describedabove. The average luminance was 3395 cd/m²; the VESA 9 pt luminanceuniformity for the entire system was approximately 54%; and the VESA 9pt color nonuniformity was 0.0287. The luminance cross section at thelateral line along the long-direction of the backlight is shown in FIG.19 as curve 1902. For the graph, the data has been smoothed by applyinga 2 mm diameter averaging filter. On this graph, the light source islocated at 0 mm.

Example 16 ARF-89 Front Reflector and BESR Back Reflector

The output surface of the backlight was covered by an ARF-89 film as thefront reflector, which was held in place against the rim of thebacklight with double-stick tape. For the back reflector, the back ofthe backlight was covered with BESR.

All of the LEDs were powered and the luminance was measured. Theillumination appeared very uniform across the width and the length ofthe backlight. The color also appeared uniform across the backlight. Acolorimetric image of the backlight was recorded using the PM 9913E-1colorimetric camera as described above. The average luminance was 3007cd/m²; the VESA 9 pt luminance uniformity for the entire system wasapproximately 83%; and the VESA 9 pt color nonuniformity was 0.0161. Theluminance cross section at the lateral line along the long-direction ofthe backlight is shown in FIG. 19 as curve 1904. For the graph, the datahas been smoothed by applying a 2 mm diameter averaging filter. On thisgraph, the light source is located at 0 mm.

Example 17 ARF-68 Front Reflector and BESR Back Reflector

The output surface of the backlight was covered by an ARF-68 film toform the front reflector, which was held in place against the rim of thebacklight with double-stick tape. The back reflector included BESR.

All of the LEDs were powered and the luminance was measured. Theillumination appeared uniform across the width and the length of thebacklight, as did the color. A colorimetric image of the backlight wasrecorded using the PM 9913E-1 colorimetric camera as described above.The average luminance was 4225 cd/m²; the VESA 9 pt luminance uniformityfor the entire system was approximately 80%; and the VESA 9 pt colornonuniformity was 0.0180. The luminance cross section at the lateralline along the long-direction of the backlight is shown in FIG. 19 ascurve 1906. For the graph, the data has been smoothed by applying a 2 mmdiameter averaging filter. On this graph, the light source is located at0 mm.

Example 18 4×ARF Front Reflector and BESR Back Reflector

The front reflector included 4×ARF that was held in place against therim of the backlight with double-stick tape. The back reflector includedBESR.

All of the LEDs were powered and the luminance was measured. Theillumination appeared uniform across the width and the length of thebacklight, as did the color. A colorimetric image of the backlight wasrecorded using the PM 9913E-1 colorimetric camera as described above.The average luminance was 4921 cd/m²; the VESA 9 pt luminance uniformityfor the entire system was approximately 79%; and the VESA 9 pt colornonuniformity was 0.0143. The luminance cross section along the lateralline along the long-direction of the backlight is shown in FIG. 19 ascurve 1908. For the graph, the data has been smoothed by applying a 2 mmdiameter averaging filter. On this graph, the light source is located at0 mm.

Example 19 4×ARF Front Reflector and 2×TIPS Back Reflector

The output surface of the backlight was covered by 4×ARF, which was heldin place against the rim of the backlight with double-stick tape to formthe front reflector. The back of the backlight was covered with a 2×TIPSto form the back reflector.

All of the LEDs were powered and the luminance was measured. Theillumination was not uniform and appeared brighter at the end closest tothe LEDs. The color appeared uniform across most of the backlight exceptfor a few narrow stripes of color observed along one side of thebacklight near the LEDs. A colorimetric image of the backlight wasrecorded using the PM 9913E-1 colorimetric camera as described above.The average luminance was 5398 cd/m²; the VESA 9 pt luminance uniformityfor the entire system was approximately 63%; and the VESA 9 pt colornonuniformity was 0.0163.

Example 20 ARF-89 Front Reflector and BESR Back Reflector

The output region of the backlight was covered by ARF-89. The film washeld in place against the rim of the backlight with double-stick tape toform the front reflector. The back of the backlight was covered withBESR to form the back reflector.

The green LEDs in the backlight were divided into two independent banksthat could be powered separately. All of the green LEDs on one side ofthe lateral line of the backlight were connected to one power circuitand all of the green LEDs on the other side of the lateral line of thebacklight were connected to a different power circuit. For this Example,both banks of green LEDs were powered and the luminance was measured.The entire output area appeared uniformly illuminated. A colorimetricimage of the backlight was recorded using the PM 9913E-1 colorimetriccamera as described above. The average luminance was 1985 cd/m² and theVESA 9 pt luminance uniformity for the entire system was approximately79%.

Example 21 ARF-89 Front Reflector and BESR Back Reflector

The output region of the backlight was covered by ARF-89. The film washeld in place against the rim of the backlight with double-stick tape toform the front reflector. The back of the backlight was covered withBESR to form the back reflector.

One bank of green LEDs was powered and the other was left off. Acolorimetric image of the backlight was recorded using the PM 9913E-1colorimetric camera as described above. The luminance was measured. Theaverage luminance was 963 cd/m² and the VESA 9 pt luminance uniformityfor the entire system was approximately 74%. The luminance decreased by50% because only half of the light sources were on, yet the uniformitybarely changed. This indicates that the backlight provided excellentlight spreading.

Comparative Example 4 APF/BGD Front Reflector and ESR Back Reflector

The output region of the backlight was covered with two films thatformed the front reflector. The first film was BGD with the beadedsurface facing the back reflector of the backlight. The second film wasAPF that was held in place against the rim of the backlight withdouble-stick tape. The back of the backlight was covered with ESR toform the back reflector.

All of the LEDs were powered and the luminance was measured. Thebacklight output was more uniform than Comparative Example 2 but stillappeared brighter at the end closest to the LEDs, getting dimmer nearthe middle and then brighter again at the distal end. The colorvariation observed in Comparative Example 2 was still visible. Acolorimetric image of the backlight was recorded using the PM 9913E-1colorimetric camera as described above. The average luminance was 3415cd/m²; the VESA 9 pt luminance uniformity for the entire system wasapproximately 74%; and the VESA 9 pt color nonuniformity was 0.0271.Note that the luminance uniformity was improved relative to ComparativeExample 2, but the color uniformity did not improve.

Example 22 ARF-68/BGD Front Reflector and ESR Back Reflector

The output region of the backlight was covered with two films thatformed the front reflector. The first film was BGD with the beadedsurface facing the back reflector of the backlight. The second film wasARF-68. The beaded gain diffuser and the asymmetric reflective film wereheld in place against the rim of the backlight with double-stick tape.The back of the backlight was covered with ESR to form the backreflector.

All of the LEDs were powered and the luminance was measured. Theillumination appeared uniform across the length and width of thebacklight, as did the color. A colorimetric image of the backlight wasrecorded using the PM 9913E-1 colorimetric camera as described above.The average luminance was 3881 cd/m²; the VESA 9 pt luminance uniformityfor the entire system was approximately 83%; and the VESA 9 pt colornonuniformity was 0.0159.

Example 23 ARF-68/BGD Front Reflector and ESR Back Reflector

The output surface of the backlight was covered with two films. Thefirst film was BGD with the beaded surface facing away from the backreflector (toward the second film). The second film was ARF-68, whichwas held in place against the rim of the backlight with double-sticktape to form the front reflector. The back of the backlight was coveredwith ESR to form the back reflector.

All of the LEDs were powered and the luminance was measured. Theillumination appeared uniform across the length and width of thebacklight, as did the color. A colorimetric image of the backlight wasrecorded using the PM 9913E-1 colorimetric camera as described above.The average luminance was 3868 cd/m²; the VESA 9 pt luminance uniformityfor the entire system was approximately 83%; and the VESA 9 pt colornonuniformity was 0.0164.

Example 24 ARF-68 Front Reflector and ESR/BGD Back Reflector

The output region of the backlight was covered with ARF-68, which washeld in place against the rim of the backlight with double-stick tape toform the front reflector. The back of the backlight was covered withESR, and a sheet of beaded gain diffuser (Opalus BS-702) was placed ontop of the specular mirror with the beaded surface facing the frontreflector. The ESR and the BGD formed the back reflector.

All of the LEDs were powered and the luminance was measured. Theillumination was not uniform and appeared brighter at the end closest tothe LEDs; however, the color appeared uniform across the backlight. Acolorimetric image of the backlight was recorded using the PM 9913E-1colorimetric camera as described above. The average luminance was 3871cd/m²; the VESA 9 pt luminance uniformity for the entire system wasapproximately 60.7%; and the VESA 9 pt color nonuniformity was 0.0163.

304 mm×152 mm Zoned Direct-Lit Backlight

The direct-lit backlight of the following examples was a rectangular boxabout 304 mm×152 mm and having an internal thickness of about 40 mm.

The backlight was illuminated by 2 red LEDs (Lumileds LXHL-PD09), 4green LEDs (Lumileds LXHL-PM09), and 2 blue LEDs (Lumileds LXHL-PR09)for a total of 8 LEDs. The LEDs were arranged in two separate clusterson small circuit boards. Each cluster used one red, two green, and oneblue LED arranged in a diamond pattern with the red and blue LEDsseparated by about 10 mm and the green LEDs separated by about 16 mm.One cluster of LEDs was centered between the long sides and 76 mm fromone of the short sides. The other cluster was centered between the longsides and 229 mm from the same short side. The circuit boards wereaffixed to an aluminum support plate and the sidewalls were attached tothis same plate.

The LEDs were powered by a custom constant-current drive circuit. Eachcluster was driven by a separate circuit so that they could be poweredindependently. The circuit allowed for the current to each of the LEDsto be adjusted to achieve a desired color point. The current througheach color of LEDs was adjusted to set the overall light output at awhite point of approximately u′=0.210 and v′=0.473.

The LED circuit board was mounted to a metal plate to which thesidewalls of the cavity were also attached. The inner sidewalls of thebacklight were covered with ESR. A partition was placed between the longsides of the cavity to divide the backlight into two halves. Each halfwas roughly square and had inside dimensions of 152 mm×152 mm. Thepartition was made from BESR. The film was folded in half with thebeaded side facing out. Double stick tape was used to hold the partitionin the folded position. The partition was trimmed to be 40 mm tall and154 mm wide. The long sidewalls had slots cut in them to hold thepartition in the proper place.

The back reflector was formed by laminating 2×TIPS to a thinpolycarbonate plate to provide support. Holes in the back reflectorallowed the lens portion of the LEDs to protrude and the polycarbonateside of the reflector was affixed to the aluminum support plate withdouble-stick tape. The highly reflective diffuse material of the backreflector extended beyond the edge of the polycarbonate plate and curvedslightly up on to the sidewalls to minimize light leaks.

The top of the backlight cavity was open. The thickness of the cavitywas determined as the distance from the inside surface of the backreflector to the top edge of the sidewalls.

Uniformity measurements were conducted using the PM-9913E colorimetriccamera in the manner described above. The backlight was mountedhorizontally at a distance of 0.5 m from the camera. A Sigma 50 mm lensat f/11 was used to collect the images.

Comparative Example 5 Diffuser Plate and 2×TIPS Back Reflector

The output surface of the backlight was covered by a diffuser plate(DR-65C, available from Astra Products, Baldwin, N.Y.).

The two halves of the backlight were divided by a partition as describedabove; however, the partition was only 35 mm high and positioned totouch the back reflector.

Both clusters were powered and the luminance was measured. A largevariation in luminance was observed across the output area. The VESA 9pt luminance uniformity for the entire system was approximately 53%. TheVESA 9 pt luminance uniformity was also calculated for the two halvesindividually (e.g., the 9 pts were all located within the square regionformed by the sidewalls and the partition). The VESA 9 pt luminanceuniformity for one side was approximately 21% and for the other side wasapproximately 23%. The luminance cross section is shown in FIG. 20. Forthe graph, the data has been smoothed by applying a 2 mm wide averagingfilter. On this graph, the partition is located at 0 mm.

Example 25 Bead-Coated ARF-84 Front Reflector and 2×TIPS Back Reflector

The diffuser plate used in Comparative Example 5 was removed and theoutput surface of the backlight was covered by ARF-84. The film alsoincluded a bead coating on the output side of the film that was formedusing the techniques described herein. Double-stick tape was used aroundthe top of the sidewalls to hold the film in place, thereby forming thefront reflector.

A circle of APF was placed over each cluster of LEDs in the backlight.The circle was slightly larger than the cluster diameter (approximately20 mm) and the pass axis was oriented so that it was orthogonal to thepass axis of the front reflector.

Both clusters were powered and the luminance was measured. The entireoutput area appeared uniformly illuminated. The VESA 9 pt luminanceuniformity for the entire system was approximately 88%. The VESA 9 ptluminance uniformity was also calculated for the two halves individually(e.g., the 9 pts were all located within the square region formed by thesidewalls and the partition). The VESA 9 pt luminance uniformity for oneside was approximately 89% and for the other side was approximately 88%.The luminance cross section is shown in FIG. 21. For the graph, the datahas been smoothed by applying a 2 mm wide averaging filter. On thisgraph, the partition is located at 0 mm.

Example 26 Bead-Coated ARF-84 Front Reflector and 2×TIPS Back Reflector

The system from Example 25 was used. One cluster of LEDs was turned onand the other one was turned off. The lit side of the backlight appeareduniformly illuminated and the other side appeared uniformly dark. Thelit side had an average luminance of approximately 869 cd/m² and thedark side had an average luminance of approximately 22 cd/m². Within thelit region (applying the analysis to only the lit half of thebacklight), the VESA 9 pt luminance uniformity was approximately 87%.Within the dark region (applying the analysis to only the dark half ofthe backlight), the VESA 9 pt luminance uniformity was approximately62%. The luminance cross section is shown in FIG. 22. For the graph, thedata has been smoothed by applying a 2 mm wide averaging filter. On thisgraph, the partition is located at 0 mm.

263 mm×147 mm Direct-Lit Backlight

The direct-lit backlight for the following examples included arectangular box about 263 mm×147 mm and had an internal thickness ofabout 18 mm.

The backlight was illuminated by 66 red LEDs (Nichia Rigel NFSR036C), 66green LEDs (Nichia Rigel NFSG036B), and 66 blue LEDs (Nichia RigelNFSB036B) for a total of 198 LEDs. The LEDs were arranged on a circuitboard in red-green-blue clusters on a square lattice with 25 mmcenter-to-center spacing between the clusters. The LEDs were powered bya custom constant-current drive circuit. All of the LEDs of a givencolor were driven at approximately the same current. The current througheach color of LEDs was adjusted to set the overall light output at awhite point of approximately u′=0.209 and v′=0.476. The currents used(after 30 minutes to reach thermal equilibrium) were 29.5 mA for red,28.8 mA for green and 7.4 mA for blue.

The LED circuit board was mounted to a metal plate to which thesidewalls of the cavity were also attached. The inner sidewalls of thebacklight were covered with ESR. The portion of the circuit board thatfaced the inside of the backlight cavity was covered with 2×TIPS. Thismaterial served as the back reflector for the cavity and extended totouch the sidewalls to minimize light leaks.

The top of the backlight cavity was covered with a diffuser plate(DR-55C, 2.0 mm thick, available from Astra Products, Baldwin, N.Y.).The thickness of the backlight cavity was determined as the distancefrom the inside surface of the back reflector to the bottom of thediffuser plate.

Uniformity measurements were conducted using the PM-9913E colorimetriccamera in the manner described above. The backlight was mountedvertically at a distance of 5 m from the camera. A Nikon 300 mm lens atf/4 was used along with the internal ND2 filter to collect the images.

Comparative Example 6 Diffuser Plate and 2×TIPS Back Reflector

The illumination output area of the direct-lit backlight was covered bya diffuser plate (DR-55C). The backlight was not uniform and showed aperiodic variation in brightness with the same pattern as the LEDs.

The average luminance (measured through the absorbing polarizer) was 969cd/m². The VESA 9 pt luminance uniformity was not a useful measure forcapturing the periodic nonuniformity. The luminance cross section isshown in FIG. 23 taken at a position directly over a row of LEDs. Forthe graph, the data has been smoothed by applying a 2 mm wide averagingfilter.

Comparative Example 7 DBEF with Diffuser Plate

The output surface of the direct-lit backlight was covered by a diffuserplate (DR-55C). A layer of DBEF-D400 (available from 3M Company) wasplaced over the diffuser plate. The backlight was more uniform than inComparative Example 6, but still showed a periodic variation inbrightness with the same pattern as the LEDs.

The average luminance (measured through the absorbing polarizer) was1543 cd/m², representing a gain of 1.59 over the diffuser plate onlybacklight in Comparative Example 6. The VESA 9 pt luminance uniformitywas not a useful measure for capturing the periodic nonuniformity. Theluminance cross section is shown in FIG. 24 taken at a position directlyover a row of LEDs. For the graph, the data has been smoothed byapplying a 2 mm wide averaging filter.

Example 27 Diffuser Plate and ARF-37 Front Reflector and 2×TIPS BackReflector

The output surface of the direct-lit backlight was covered by a diffuserplate (DR-55C). ARF-37 laminate between 2 layers of polycarbonate (oneside used 5 mil PC with 12% haze facing out and the other side used 5mil PC with 20% haze facing out) was placed over the diffuser plate toform the front reflector. The back reflector was formed from 2×TIPS. Thebacklight was more uniform than in Comparative Example 6, and theperiodic variation in brightness was less visible.

The average luminance (measured through the absorbing polarizer) was1555 cd/m², representing a gain of 1.60 over the diffuser plate only inComparative Example 6. The VESA 9 pt luminance uniformity was not auseful measure for capturing the periodic nonuniformity. The luminancecross section is shown in FIG. 25 taken at a position directly over arow of LEDs. For the graph, the data has been smoothed by applying a 2mm wide averaging filter.

Example 28 Diffuser Plate and 3×ARF Front Reflector and 2×TIPS BackReflector

The output surface of the direct-lit backlight was covered by a diffuserplate (DR-55C). 3×ARF was placed over the diffuser plate to form thefront reflector. The back reflector was formed from 2×TIPS. Thebacklight was more uniform than in Example 25, and the periodicvariation in brightness was difficult to see.

The average luminance (measured through the absorbing polarizer) was1628 cd/m², representing a gain of 1.68 over the diffuser plate only inComparative Example 6. The VESA 9 pt luminance uniformity was not auseful measure for capturing the periodic nonuniformity. The luminancecross section is shown in FIG. 26 taken at a position directly over arow of LEDs. For the graph, the data has been smoothed by applying a 2mm wide averaging filter.

Comparative Example 8 Diffuser Plate, BGD/BEF/APF Front Reflector and2×TIPS Back Reflector

The output surface of the backlight was covered by a diffuser plate(DR-55C). A three-film stack was placed on top of the diffuser plate.The stack included (in order from the diffuser plate out toward theviewer) BGD, a prism film (BEF III-10T, available from 3M Company), andAPF. The APF was laminated between 2 layers of polycarbonate (one sideused 5 mil PC with 12% haze facing out and the other side used 5 mil PCwith 20% haze facing out).

The average luminance (measured through the absorbing polarizer) was1951 cd/m², representing a gain of 2.01 over the diffuser plate only inComparative Example 6. The VESA 9 pt luminance uniformity was not auseful measure for capturing the periodic nonuniformity. The luminancecross section is shown in FIG. 27 taken at a position directly over arow of LEDs. For the graph, the data has been smoothed by applying a 2mm wide averaging filter.

Example 29 23″ LCD-TV

A commercial, CCFL-backlit 23″ LCD-TV (model LNS2341 WX/XAA fromSamsung) was modified to include an LED backlight.

The backlight was illuminated by 264 red LEDs (Osram LR G6SP-CADB), 264green LEDs (Osram LT G6SP-CBEB), and 264 blue LEDS (Osram LB G6SP-V2BB)for a total of 792 LEDs. The LEDs were modified by forming a dome ofencapsulant over the output area of the LEDs. The dome was made usingphotocurable silicone (3M PSE-002, available from 3M Company). Acontrolled amount of the silicone was dispensed from a syringe onto theLED to form a hemispherical drop. The drop was then quickly cured usinga high-power UV lamp to retain the high dome shape. The dome shapeincreases the efficiency of the LEDs without much change in the emissionpattern.

The LEDs were arranged on circuit boards in red-green-blue clusters on asquare lattice with approximately 25 mm center-to-center spacing betweenthe clusters. Each circuit board had 6 rows of 11 clusters and four ofthese boards were used to cover the area of the backlight. The TV wasdisassembled and the CCFL bulbs and associated circuits were removed.The boards were assembled into the metal backlight housing. ESR wasplaced over the circuit board with holes provided for the LEDs to formthe back reflector. The sidewalls of the backlight housing were coveredwith 2×TIPS. The diffuser plate from the original TV was placed over thebacklight. A second diffuser plate (DR-55C, 2.0 mm thick) was placed onthe first diffuser.

The film stack was BGD, BEF III-10T, and ARF-68 (with polycarbonatelaminated to both sides (0.127 mm thick Iupilon™ Film, Grade FE-2000 M05from Mitsubishi Engineering-Plastics Corp., Tokyo, Japan, with thetextured surface facing the film). The LED backlight had a internalthickness (space between the back reflector and the bottom of the firstdiffuser plate) of about 16 mm. The backlight was replaced and the TVwas reassembled.

All of the LEDs of one color were connected in a series-parallelconfiguration with 11 LEDs in series strings and 24 strings of LEDs (6from each of 4 boards) connected in parallel. The LEDs were poweredusing external laboratory power supplies (Tenma 72-6615) inconstant-current control. The circuit of red LEDs was driven at 0.60 A,20.1 V (for an average current of 25 mA per red LED), the circuit ofgreen LEDs was driven at 1.06 A, 31.6V (for an average current of 44 mAper green LED), and the circuit of blue LEDs was driven at 0.50 A, 30.3V(for an average current of 21 mA per blue LED). These currents weremeasured after the fully assembled TV was powered for about 1.5 hours towarm up.

Uniformity measurements were conducted using the Prometric camera in themanner described herein. The display was mounted vertically at adistance of 5 m from the camera. A Sigma 105 mm lens at f/11 was used tocollect the images. For the uniformity measurement, a white screen wasdisplayed on the TV using a personal computer attached to the PC inputport of the TV.

The display appeared uniform in luminance and color. The averageluminance (measured through the panel) was 428 cd/m²; the VESA 9 ptluminance uniformity was approximately 83%; and the VESA 9 pt colornonuniformity was 0.0097.

Example 30 300 mm×300 mm Zoned Direct-lit Backlight

A 12″×12″ zoned Backlight included a rectangular box about 300 mm×300 mmand had an internal thickness of about 25 mm.

The backlight was illuminated by 4 red LEDs (Lumileds UCHL-DD09), 8green LEDs (Lumileds LXHL-DM09), and 4 blue LEDs (Lumileds LXHL-DR09)for a total of 16 LEDs. The LEDs were arranged in four separate clusterson small circuit boards. Each cluster used one red, two green, and oneblue LED arranged in a diamond pattern with the red and blue LEDsseparated by about 10 mm and the green LEDs separated by about 16 mm.

A cluster of LEDs was located at the center of each of the four 150mm×150 mm quadrants of the backlight. The circuit boards were affixed toan aluminum support plate and the sidewalls were attached to this sameplate.

The LEDs were powered by a custom constant-current drive circuit. Eachcluster was driven by a separate circuit so that they could be poweredindependently. The circuit allowed for the current to each of the LEDsto be adjusted to achieve a desired color point. The current througheach color of LEDs was adjusted to set the overall light output at awhite point of approximately u′=0.179 and v′=0.438.

The LED circuit board was mounted to a metal plate to which thesidewalls of the cavity were also attached. The inside of the sidewallsof the backlight was covered with ESR. The backlight was separated intofour quadrants by two orthogonal partitions. The partitions were madefrom aluminum sheet about (about 1/16″ thick) that was covered on bothsides with BESR. The partitions were notched halfway through at themiddle of the span to allow them to interlock at the intersection in themiddle of the backlight. The long sidewalls had slots cut in them tohold the partition in the proper place. There was a small gap (about 2mm) between the top of the partition and the top film stack.

The back reflector was formed using 2×TIPS attached to a thinpolycarbonate plate to provide support. Holes in the back reflectorallowed the lens portion of the LEDs to protrude and the polycarbonateside of the reflector was affixed to the aluminum support plate withdouble-stick tape. The 2×TIPS extended beyond the edge of thepolycarbonate plate and curved slightly up on to the sidewalls tominimize light leaks.

A partial reflector layer was placed over the top of the backlightcavity. The partial reflector was two sheets of ARF-84/BGD laminated (onthe non-coated side) to 5 mil PC with 20% haze. Both sheets wereoriented so that the bead coatings faced away from the light sources.The two sheets were placed in physical contact, but were not laminatedto one another with the polarization axes aligned. A clear acrylic plate(approximately 2 mm thick) was placed over the films to help hold themin place.

A circle of APF was placed over each cluster of LEDs. The circle wasslightly larger than the cluster diameter (approximately 25 mm) and thepolarization axis was oriented so that it was perpendicular to thepolarization axis of the partial reflector layer.

The thickness of the backlight cavity (the distance from the insidesurface of the back reflector to the top edge of the sidewalls) wasapproximately 25 mm.

Uniformity measurements were conducted using the PR-9913E colorimetriccamera in the manner described above. The display was mounted verticallyat a distance of 5 m from the camera. A Nikon 300 mm lens at f/11 wasused to collect the images. No absorbing polarizer was used for thesemeasurements.

All four clusters were powered and the luminance was measured. Theentire output area appeared uniformly illuminated. The average luminancewas 659 cd/m². The VESA 9 pt luminance uniformity for the entire systemwas approximately 86%. The VESA 9 pt luminance uniformity was alsocalculated for each of the four quadrants individually (e.g., each ofthe 9 pts were all located within the square regions formed by thesidewalls and the partition). The VESA 9 pt luminance uniformity foreach quadrant was 88%, 94%, 86%, and 90%.

Example 31 150 mm×150 mm Direct-lit Backlight

The following example utilized a 150 mm×150 mm backlight that had aninternal thickness of about 30 mm.

The backlight was illuminated by 13 red LEDs (Nichia NSSR100B), 26 greenLEDs (Nichia NSSG100B), and 13 blue LEDS (Nichia NSSB 100B) for a totalof 52 LEDs. The LEDs were arranged on circuit boards inred-green-green-blue clusters in five rows. Within a row the clusterswere spaced approximately 50 mm apart and the rows were separated byabout 25 mm. The first row had three clusters, the second row had twoclusters, the third row had three clusters, the fourth row had twoclusters and the fifth row had three clusters. The clusters within eachrow were uniformly distributed about the centerline of the circuitboard.

The LED circuit board was mounted to a metal plate to which thesidewalls of the cavity were also attached. MCPET was placed over thecircuit board with holes provided for the LEDs. The inside of thesidewalls of the backlight were covered with 2×TIPS.

The LEDs were powered by a custom constant-current drive circuit. Eachcluster was driven by a separate circuit so that they could be poweredindependently. The circuit allowed for the current to each of the LEDsto be adjusted to achieve a desired color point. The current througheach color of LED was adjusted to set the overall light output at awhite point of approximately u′=0.181 and v′=0.461.

A diffuser plate (DR-55C, 2.0 mm thick) was placed over the top of thebacklight cavity. 2×ARF (laminated between two layers of 5 mil PC) wasplaced over the diffuser plate to form the front reflector.

Uniformity measurements were conducted using the Prometric camera in themanner described above. The display was mounted vertically at a distanceof 5 m from the camera. A Nikon 300 mm lens at f/11 was used to collectthe images. No absorbing polarizer was used for these measurements.

The backlight appeared uniform in luminance and color. The averageluminance was 559 cd/m²; the VESA 9 pt luminance uniformity wasapproximately 90%; and the VESA 9 pt color nonuniformity was 0.0068.

Unless otherwise indicated, references to “backlights” are also intendedto apply to other extended area lighting devices that provide nominallyuniform illumination in their intended application. Such other devicesmay provide either polarized or unpolarized outputs. Examples includelight boxes, signs, channel letters, and general illumination devicesdesigned for indoor (e.g. home or office) or outdoor use, sometimesreferred to as “luminaires.”

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure, except tothe extent they may directly contradict this disclosure. Illustrativeembodiments of this disclosure are discussed and reference has been madeto possible variations within the scope of this disclosure. These andother variations and modifications in the disclosure will be apparent tothose skilled in the art without departing from the scope of thedisclosure, and it should be understood that this disclosure is notlimited to the illustrative embodiments set forth herein. Accordingly,the disclosure is to be limited only by the claims provided below.

What is claimed is:
 1. A backlight, comprising: a front reflector and aback reflector that form a hollow light recycling cavity comprising anoutput surface; and one or more light sources disposed to emit lightinto the light recycling cavity; wherein the front reflector comprises afirst asymmetric reflective film comprising alternating polymer layers,and further wherein the front reflector comprises an on-axis averagereflectivity of at least 90% for visible light polarized in a firstplane, and an on-axis average reflectivity of at least 25% but less than90% for visible light polarized in a second plane perpendicular to thefirst plane.
 2. The backlight of claim 1, wherein the one or more lightsources comprise one or LEDs.
 3. The backlight of claim 1, wherein theback reflector comprises an on-axis average reflectivity of at least 95%for visible light of any polarization.
 4. The backlight of claim 1,wherein the front reflector comprises a first on-axis averagetransmissivity for visible light polarized in the first plane, and asecond on-axis average trausmissivity for visible light polarized in thesecond plane, and further wherein a ratio of the second on-axistransmissivity to the first on-axis transmissivity is at least
 10. 5.The backlight of claim 4, wherein the ratio is at least
 20. 6. Thebacklight of claim 1, wherein p-polarized visible light that ispolarized in the second plane exhibits a substantially constant averagereflectivity as angle of incidence with the front reflector increasesfrom near zero degrees to 60 degrees.
 7. The backlight of claim whereinp-polarized visible light that is polarized in the second plane exhibitsan increasing average reflectivity as angle of incidence with the frontreflector increases from near zero degrees to 60 degrees.
 8. Thebacklight of claim 7, wherein the increasing average reflectivity of thep-polarized light is substantially similar to an increasing averagereflectivity of s-polarized visible light that is polarized in thesecond plane as angle of incidence with the front reflector increasesfrom near zero degrees to 60 degrees.
 9. The backlight of claim 1,wherein the front reflector further comprises a second asymmetric,reflective film comprising alternating polymer layers positionedproximate the first asymmetric reflective film.
 10. The backlight ofclaim 9, wherein the first asymmetric reflective film is attached to thesecond asymmetric reflective film using an optical adhesive.
 11. Thebacklight of claim 1, wherein at least one of the front and backreflectors reflects light semi-specularly.
 12. The backlight of claim 1,further comprising, side reflectors disposed about the periphery of thehollow light recycling cavity to substantially enclose the recyclingcavity, the side reflectors having an on-axis average reflectivity of atleast 95% for visible light of any polarization.
 13. A display system,comprising: a display panel; and a backlight disposed to provide lightto the display panel, the backlight comprising: a front reflector and aback reflector that form a hollow light recycling cavity comprising anoutput surface; and one or more light sources disposed to emit lightinto the light recycling cavity; wherein the front reflector comprises afirst asymmetric reflective film comprising alternating polymer layers,and further wherein the from reflector comprises an on-axis averagereflectivity of at least 90% for visible light polarized in a firstplane, and an on-axis average reflectivity of at least 25% but less than90% for visible light polarized in a second plane perpendicular to thefirst plane.